MaximilianW, thanks for your reply.
Eventually the problem was solved by recover the machine from a backup, and ïnstall the update.
Probably the update wasn't the cause. (The only relation with the update was the moment the problem occured.)
Since the 20th of februari I experience problems with a couple of PHP-based websites. They are hosted on Windows 2012 server with Plesk 11.5.30, and the sites are installations of Feng Office.
After troubleshooting this problem, I noticed that the only change during the period since last...