Index: /trunk/src/org/openstreetmap/josm/io/session/SessionWriter.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/io/session/SessionWriter.java	(revision 14724)
+++ /trunk/src/org/openstreetmap/josm/io/session/SessionWriter.java	(revision 14725)
@@ -222,5 +222,5 @@
             }
             // path.toString() returns backslashes on Windows, see #17228
-            return StreamUtils.toStream(path)
+            return (isZip() ? "../" : "") + StreamUtils.toStream(path)
                     .map(Object::toString)
                     .collect(Collectors.joining("/"));