I don't know the specific source of the problem as you'd need to trap it at the tag/servlet level. It looks like it is Struts related. The iterator used for accessing/consuming a collection is throwing an exception because the source collection is being modified before the iterator is finished with the collection.
List list = new ArrayList(Arrays.asList(args));
Iterator i = list.iterator();
This example throws this type of exception. I'm sure you will be able to recognise it when you see it. Otherwise, if it is embedded in the Struts tag, you may want to check for any Strut bug fixes/bug reports.