Index: trunk/autoquest-core-tasktrees/src/main/java/de/ugoe/cs/autoquest/tasktrees/treeimpl/Task.java
===================================================================
--- trunk/autoquest-core-tasktrees/src/main/java/de/ugoe/cs/autoquest/tasktrees/treeimpl/Task.java	(revision 2132)
+++ trunk/autoquest-core-tasktrees/src/main/java/de/ugoe/cs/autoquest/tasktrees/treeimpl/Task.java	(revision 2136)
@@ -163,4 +163,10 @@
     @Override
     public Collection<ITaskInstance> getInstances() {
+        // the feature of an unmodifiable collection was added later. To ensure backward
+        // compatibility with stored data, check if the object exists, if not return the instances.
+        if (unmodifiableInstances == null) {
+            return instances;
+        }
+        
         return unmodifiableInstances;
     }