Looks like the server isn't liking the "getimagesize" command - not sure why a server would do that. Unless there are directory restrictions in place?
The changebasic uses the same 'getimagesize' command if you are allowing users to upload avatars.
http://www.php.net/getimagesize"Unable to open 'http://80.126.87.6/ubbthreads/bestanden/3869-logo_sm.gif' for reading"
Leads me to believe that it's a permissions thing? Is PHP in safe mode?
I notice in changebasic.php Scream adds a @ sign before it. I don't really know what that does.

Try in showflat and show threaded changing
getimagesize
to
@getimagesize
See if that helps.