Use case was to simply create a user-group which failed.
Looking at the code, the part failing:

                $result&=change_group_status($usrgrpid,$users_status);
                $result&=change_group_gui_access($usrgrpid,$gui_access);
                if(!$result) return     $result;

in frontends/php/include/users.inc.php.

The reason is because pg_query in db.inc.php will return a resource ID (integer ?) which may return false on a binary AND. In my case, the previous value of $result was resource 127 and the return of change_group_status() was 128. Doing a binary AND between 127 and 128 returned false.

In my case, I patched the file users.inc.php, but you may just want to patch DBexecute to force it to return a boolean true/false.