There is a bug in moodle/course/moodleform_mod.php https://tracker.moodle.org/browse/MDL-56617 when CBM grading is used. Grades greater than 100 may not be entered, even though with CBM a maxium of 300 is possible.
The only other place where the same error message is generated is in moodle/mod/workshop/mod_form.php. Hence my question: is it at all possible that CBM grading would be used in workshop? On a cursory inspection, this does not seem to be the case.
Irrespective of that, there is a pending request https://tracker.moodle.org/browse/MDL-58223 to set a maximum grade greater than 100. If this goes ahead, it will probably encounter the same problem as MDL-56617, albeit for a different reason.