Ah, that explains. You will have to upgrade to the recent Moodle 1.9.x to have this fixed.
In Moodle 2.0, teachers can switch workshop into so called set-up phase where students can not do anything. Also, even if the workshop is in submission phase or assessment phase but it is before/after the deadlines defined in Access control part of the activity settings, students can not do anything.
workshop_list_teacher_submissions: unable to get submission
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