Hello,
This is not exactly an issue with ProjeQtOr : that only means you're trying to display too many fields on your timesheet page for this user.
You can either change the value in the php.ini file as suggested in the warning or you can also check if this user has the right activities to fill in and if he really needs all of them at the same time (too many lines to search in may not be really efficient when it comes to fill in the time spent).
Also, you may try to switch from the month view to the week view (less fields to display).