Index: trunk/test/unit/org/openstreetmap/josm/testutils/PluginServer.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/testutils/PluginServer.java	(revision 14168)
+++ trunk/test/unit/org/openstreetmap/josm/testutils/PluginServer.java	(revision 14179)
@@ -26,4 +26,5 @@
 import com.github.tomakehurst.wiremock.junit.WireMockRule;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Streams;
 
 public class PluginServer {
@@ -118,5 +119,6 @@
 
                 if (jarPath.startsWith(filesRootPath)) {
-                    return filesRootPath.relativize(jarPath).toString();
+                    // would just use .toString() but need to force use of *forward slash* path separators on all platforms
+                    return Streams.stream(filesRootPath.relativize(jarPath)).map(p -> p.toString()).collect(Collectors.joining("/"));
                 }
             }