diff --git src/org/openstreetmap/josm/gui/bugreport/BugReportDialog.java src/org/openstreetmap/josm/gui/bugreport/BugReportDialog.java
index af74571464..0f0ff68af2 100644
--- src/org/openstreetmap/josm/gui/bugreport/BugReportDialog.java
+++ src/org/openstreetmap/josm/gui/bugreport/BugReportDialog.java
@@ -31,6 +31,7 @@ import org.openstreetmap.josm.spi.preferences.Config;
 import org.openstreetmap.josm.tools.GBC;
 import org.openstreetmap.josm.tools.ImageProvider;
 import org.openstreetmap.josm.tools.InputMapUtils;
+import org.openstreetmap.josm.tools.Utils;
 import org.openstreetmap.josm.tools.bugreport.BugReport;
 import org.openstreetmap.josm.tools.bugreport.BugReportQueue.SuppressionMode;
 import org.openstreetmap.josm.tools.bugreport.BugReportSender;
@@ -45,6 +46,8 @@ import org.openstreetmap.josm.tools.bugreport.ReportedException;
  */
 public class BugReportDialog extends JDialog {
     private static final int MAX_MESSAGE_SIZE = 500;
+    /** whether the user has been told about file names that cannot be represented, which is done once per session */
+    private static volatile boolean unmappableFileNameExplained;
     // This is explicitly not an ExtendedDialog - we still want to be able to display bug reports if there are problems with preferences/..
     private final JPanel content = new JPanel(new GridBagLayout());
     private final BugReport report;
@@ -228,6 +231,22 @@ public class BugReportDialog extends JDialog {
                     JOptionPane.ERROR_MESSAGE
                     );
             return SuppressionMode.NONE;
+        } else if (e.isUnmappableFileName()) {
+            // Not a bug in JOSM, but a locale which cannot represent the name of a file, see #14596. Explain how to
+            // fix it rather than asking for a bug report, and only once: browsing other folders throws it again.
+            if (!unmappableFileNameExplained) {
+                unmappableFileNameExplained = true;
+                GuiHelper.runInEDTAndWait(() -> JOptionPane.showMessageDialog(MainApplication.getMainFrame(),
+                        tr("<html>JOSM cannot handle the name of a file, because it is running with the character set {0} "
+                                + "for file names, which cannot represent it.<br>"
+                                + "This happens when JOSM is started without a UTF-8 locale, for example with {1}, "
+                                + "or with a locale which is not installed.<br><br>"
+                                + "Please start JOSM with the environment variable {2}.</html>",
+                                Utils.getSystemProperty("sun.jnu.encoding"), "<tt>LANG=C</tt>", "<tt>LC_ALL=C.UTF-8</tt>"),
+                        tr("Unsupported file name"),
+                        JOptionPane.WARNING_MESSAGE));
+            }
+            return SuppressionMode.NONE;
         } else {
             PluginDownloadTask downloadTask = PluginHandler.updateOrdisablePluginAfterException(e);
             return GuiHelper.runInEDTAndWaitAndReturn(() -> {
diff --git src/org/openstreetmap/josm/tools/bugreport/ReportedException.java src/org/openstreetmap/josm/tools/bugreport/ReportedException.java
index 28774399ee..9a2d7f4543 100644
--- src/org/openstreetmap/josm/tools/bugreport/ReportedException.java
+++ src/org/openstreetmap/josm/tools/bugreport/ReportedException.java
@@ -4,6 +4,7 @@ package org.openstreetmap.josm.tools.bugreport;
 import java.io.PrintWriter;
 import java.io.Serializable;
 import java.lang.reflect.InvocationTargetException;
+import java.nio.file.InvalidPathException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -21,6 +22,7 @@ import java.util.function.Supplier;
 
 import org.openstreetmap.josm.tools.Logging;
 import org.openstreetmap.josm.tools.StreamUtils;
+import org.openstreetmap.josm.tools.Utils;
 
 /**
  * This is a special exception that cannot be directly thrown.
@@ -277,6 +279,20 @@ public class ReportedException extends RuntimeException {
         return StreamUtils.toStream(CauseTraceIterator::new).anyMatch(OutOfMemoryError.class::isInstance);
     }
 
+    /**
+     * Check if this is caused by the name of a file which cannot be represented in the character set Java uses for
+     * file names. Java takes that character set from the locale, so this happens when JOSM runs in the C/POSIX locale,
+     * or with a locale which is not installed: file names are then ASCII only, and opening a folder which holds a
+     * non-ASCII name fails. See #14596.
+     * @return <code>true</code> if it is.
+     * @since xxx
+     */
+    public boolean isUnmappableFileName() {
+        return !"UTF-8".equalsIgnoreCase(Utils.getSystemProperty("sun.jnu.encoding"))
+                && StreamUtils.toStream(CauseTraceIterator::new).anyMatch(t -> t instanceof InvalidPathException
+                        && String.valueOf(((InvalidPathException) t).getReason()).contains("unmappable characters"));
+    }
+
     /**
      * Iterates over the causes for this exception. Ignores cycles and aborts iteration then.
      * @author Michal Zangl
diff --git test/unit/org/openstreetmap/josm/gui/bugreport/BugReportDialogTest.java test/unit/org/openstreetmap/josm/gui/bugreport/BugReportDialogTest.java
new file mode 100644
index 0000000000..086a0f5d9d
--- /dev/null
+++ test/unit/org/openstreetmap/josm/gui/bugreport/BugReportDialogTest.java
@@ -0,0 +1,52 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.gui.bugreport;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.nio.file.InvalidPathException;
+import java.util.Collections;
+
+import javax.swing.JOptionPane;
+
+import org.junit.jupiter.api.Test;
+import org.openstreetmap.josm.TestUtils;
+import org.openstreetmap.josm.testutils.mockers.JOptionPaneSimpleMocker;
+import org.openstreetmap.josm.tools.bugreport.BugReport;
+import org.openstreetmap.josm.tools.bugreport.BugReportQueue.SuppressionMode;
+import org.openstreetmap.josm.tools.bugreport.ReportedException;
+
+/**
+ * Unit tests of {@link BugReportDialog}.
+ */
+class BugReportDialogTest {
+
+    /**
+     * A file name the locale cannot represent is explained to the user, once, instead of asking for a bug report.
+     * See #14596.
+     */
+    @Test
+    void testUnmappableFileNameIsExplainedOnce() {
+        TestUtils.assumeWorkingJMockit();
+        final JOptionPaneSimpleMocker mocker = new JOptionPaneSimpleMocker(Collections.singletonMap(
+                "<html>JOSM cannot handle the name of a file, because it is running with the character set ANSI_X3.4-1968 "
+                        + "for file names, which cannot represent it.<br>"
+                        + "This happens when JOSM is started without a UTF-8 locale, for example with <tt>LANG=C</tt>, "
+                        + "or with a locale which is not installed.<br><br>"
+                        + "Please start JOSM with the environment variable <tt>LC_ALL=C.UTF-8</tt>.</html>",
+                JOptionPane.OK_OPTION));
+        final ReportedException e = BugReport.intercept(new InvalidPathException("/home/user/Capture d'écran.png",
+                "Malformed input or input contains unmappable characters"));
+        final String jnuEncoding = System.getProperty("sun.jnu.encoding");
+        try {
+            System.setProperty("sun.jnu.encoding", "ANSI_X3.4-1968");
+            // no bug report dialog: that would fail here, as the tests run headless
+            assertEquals(SuppressionMode.NONE, BugReportDialog.showFor(e, 0));
+            // browsing another folder throws it again, which is not worth another message
+            assertEquals(SuppressionMode.NONE, BugReportDialog.showFor(e, 1));
+        } finally {
+            System.setProperty("sun.jnu.encoding", jnuEncoding);
+        }
+        assertEquals(1, mocker.getInvocationLog().size());
+        assertEquals("Unsupported file name", mocker.getInvocationLog().get(0)[2]);
+    }
+}
diff --git test/unit/org/openstreetmap/josm/tools/bugreport/ReportedExceptionTest.java test/unit/org/openstreetmap/josm/tools/bugreport/ReportedExceptionTest.java
index 1125bfd5d2..36cf937fff 100644
--- test/unit/org/openstreetmap/josm/tools/bugreport/ReportedExceptionTest.java
+++ test/unit/org/openstreetmap/josm/tools/bugreport/ReportedExceptionTest.java
@@ -2,7 +2,10 @@
 package org.openstreetmap.josm.tools.bugreport;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import java.nio.file.InvalidPathException;
 import java.util.Arrays;
 
 import org.junit.jupiter.api.Test;
@@ -60,6 +63,31 @@ class ReportedExceptionTest {
         e.put("testList", Arrays.asList(a));
     }
 
+    /**
+     * Tests {@link ReportedException#isUnmappableFileName()}: a file name which the character set used for file names
+     * cannot represent is recognised, also as the cause of another exception, but only if that character set is not
+     * UTF-8 - otherwise the locale is not to blame. See #14596.
+     */
+    @Test
+    void testIsUnmappableFileName() {
+        // the exception sun.nio.fs.UnixPath throws for "Capture d'écran.png" when the locale is C or not installed
+        InvalidPathException unmappable = new InvalidPathException("/home/user/Capture d'\u00e9cran.png",
+                "Malformed input or input contains unmappable characters");
+        String jnuEncoding = System.getProperty("sun.jnu.encoding");
+        try {
+            System.setProperty("sun.jnu.encoding", "ANSI_X3.4-1968");
+            assertTrue(new ReportedException(unmappable).isUnmappableFileName());
+            assertTrue(new ReportedException(new IllegalStateException(unmappable)).isUnmappableFileName());
+            assertFalse(new ReportedException(new InvalidPathException("a\0b", "Nul character not allowed")).isUnmappableFileName());
+            assertFalse(new ReportedException(new IllegalStateException()).isUnmappableFileName());
+
+            System.setProperty("sun.jnu.encoding", "UTF-8");
+            assertFalse(new ReportedException(unmappable).isUnmappableFileName());
+        } finally {
+            System.setProperty("sun.jnu.encoding", jnuEncoding);
+        }
+    }
+
     /**
      * Tests that {@link ReportedException#isSame(ReportedException)} works as expected.
      */