I've now got it narrowed down to the following differences in contents
of the jarfiles:
--- /tmp/lap.jar 2012-07-04 16:58:46.655131809 +0200
+++ /tmp/desk.jar 2012-07-04 16:51:00.015132187 +0200
@@ -281,8 +281,6 @@
gnu/kawa/servlet/HttpOutputStream.class
gnu/kawa/servlet/HttpRequestContext.class
gnu/kawa/servlet/KawaAutoHandler.class
-gnu/kawa/servlet/KawaHttpHandler.class
-gnu/kawa/servlet/KawaHttpHandler$Context.class
gnu/kawa/servlet/ServletPrinter.class
gnu/kawa/slib/condition.class
gnu/kawa/slib/condition$Mntype.class
@@ -593,6 +591,7 @@
gnu/kawa/util/HashNode.class
gnu/kawa/util/IdentityHashTable.class
gnu/kawa/util/IntHashTable.class
+gnu/kawa/util/PreProcess.class
gnu/kawa/util/RangeTable.class
gnu/kawa/util/RunTestScript.class
gnu/kawa/util/WeakHashNode.class