user settings: forward.php etc is stuck

[Markus Heller]

Dear List,

I have a problem with the forwarding configuration of the web gui where a user 
may log into his own preferences account:

admin/user/user.php?action=modify is still ok, responsive
admin/user/forward.php is stuck, won't reply
/admin/user/deliver.php is stuck, won't reply
/admin/user/vacation.php is stuck, won't reply

and once I click on one of those links which are stuck, the first, initially 
responsive link won't work anymore either. 

Any hints how to diagnose further? It's not about the Apache, since I already 
tested this: I substituted one of the stuck files by a new file which 
contained nothing but the phpinfo() call - and this worked fine. 

Please tell me where to look for further diagnose!

Markus