Forum Particular scheduled task won't get executed
 

I installed a modification that uses a scheduled task to check for inactive users. The problem is that the task won't get automatically executed when I need it, only manual activation works. I tried to refresh my forum at the specified time but nothing happened (there is no record in the log). The run time set in the scheduled tasks manager in AdminCP just changed to the next one.

Other scheduled tasks work fine.

Any ideas what's wrong with it?
(I doubt that the modification has something to do with this problem, that's why I ask here.)