[otrs] permissions to upload file

I had to move/re-install my OTRS. All works fine (thanks to Alessandro!) :-) But I cannot upload a file as attachment. It says: Software error: Permission denied at ../..//Kernel/System/WebUploadCache.pm line 140. So, I have to chmod or chown something. But WHAT?? Please tell me which permissions to which directory or file? Thanks, Marc