The forum function get_modinfo_special has some special processing that is only meant for the OU shared activities course.

If a course has no course modules for some reason, then the current check is insufficient to distinguish between this type of course and a shared activities course.

A simple change in the code will solve this:

if (class_exists('ouflags') && !count($modinfo->cms)) {

change to

if (class_exists('ouflags') && $course->idnumber==='!sharedactivities') {