Helpdesk works again
After a day of bug hunting and help from the great guys and gals at Userscape we were able to track down the problem to a wrong 'include_path' setting in the PHP preferences. I have absolutely no clue why this setting was changed during a PHP security update or if it has been there all along. Fact is the problem occured only after the update, and only in this one application. The value of 'include_path' was set to '.' in a global confixx file 'confixx_mhost.conf'. After commenting out the property the whole thing suddenly worked again. So go ahead and use the helpdesk. Have a great day, Torsten.