Index: /trunk/src/org/openstreetmap/josm/io/NetworkManager.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/io/NetworkManager.java	(revision 16682)
+++ /trunk/src/org/openstreetmap/josm/io/NetworkManager.java	(revision 16683)
@@ -3,8 +3,9 @@
 
 import java.net.URL;
+import java.util.Collections;
 import java.util.EnumSet;
-import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.openstreetmap.josm.tools.Logging;
@@ -16,5 +17,5 @@
 public final class NetworkManager {
 
-    private static final Map<String, Throwable> NETWORK_ERRORS = new HashMap<>();
+    private static final Map<String, Throwable> NETWORK_ERRORS = new ConcurrentHashMap<>();
 
     private static final Set<OnlineResource> OFFLINE_RESOURCES = EnumSet.noneOf(OnlineResource.class);
@@ -63,5 +64,5 @@
      */
     public static Map<String, Throwable> getNetworkErrors() {
-        return new HashMap<>(NETWORK_ERRORS);
+        return Collections.unmodifiableMap(NETWORK_ERRORS);
     }