Closing Filemanager popups cause two requests and ConcurrentModificationExceptions

Since the last changes in the engine framework to allow closing with the X button, we now often get ConcurrentModificationExceptions, because on closing of each second level popup (for instance rename file, change file permissions), always two parallel HTTP requests are triggered - the query parameters of the parallel requests for instance are:

cid=8&screen=edit&fid=22&screen=edit&selectDir=/shared/Comp|C3|A9tition
cid=8&screen=edit&fid=22&screen=edit&path=/shared/Comp|C3|A9tition/altavista.gif&matrix=8207