Found the answer! The problem resulted from a choice made while installing the rubric assignment module - rather than delete or move the assignments directory I just renamed it assignment.bck. Solution was to move it to an archive directory (don't delete - you may want that later!). Further details in this post: http://moodle.org/mod/forum/discuss.php?d=143893.
Roles and permissions
Un/Assign role in course leads to blank result page
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