Update: /tmp is full, that is the reason why uploads don't work any more. Changed tmp directory to 

/var/tmp, anyways users abusing /tmp to store compute data should be .... 

We are having slight problems with the file upload!

After todays server restart, Admin and other users noticed problems with uploading any files. No idea what this is about.

Checked if the disk is full, but has 20+ GB of space free, directory is writeable to apache.

There is no message in the error log either...