I tried that, but then on nginx it does not create multiple files, it only creates one.
Alexander from woltlab looked at it and this was his conclusion:
"I've checked the cronjob (which calls the worker itself) and the issue might be the way the entire file is being created: All data is stored into a temporary variable and then written in one step only. This saves a lot of time because the disks are not busy writing every single line, but this also causes the temporary variable to eventually grow really big. Filling the buffer variable up to a certain threshold and then flushing to disk might avoid this problem."