при переносе 1.9 на новый сервер и апе системы столкнулся с этой проблемой. после долгих изысканий помогло вот это
Mikeles added a comment - 01/Jul/11 4:46 PM
Hello to all
after reviewing line by line all libraries of submissions, i found the problem
in \mod\assignment\lib.php
line 1151 (or near), we have the sql query, only change it:
COALESCE(SIGN(SIGN(s.timemarked) + SIGN(s.timemarked - s.timemodified)), 0) AS status ';
COALESCE(SIGN(CAST(s.timemarked as SIGNED) + SIGN(CAST(s.timemarked as SIGNED) - CAST(s.timemodified as SIGNED))), 0) AS status ';
i'm using mysql 5.5.8
when the clause try order this send a error because, timemarked sometimes is null and no have sign (is UNSIGNED in the DB)
regards 
