Lynn, we just cracked it.
It appears it is some kind of confusion caused by the entries for the user in the mdl_user_preferences table in the DB.
What I did was do a query for the user in question and then cleared the values for assign_filter and flextable_mod_assign_grading.
I then got the user to login and watched the values get recreated. No thing seem to be working as normal for that user
So you (or your local SQL person) runs something like this against your moodle database, it should solve it.
UPDATE [Moodle_DB].[dbo].[mdl_user_preferences]
SET value = ''
WHERE userid = [the moodle userid of the person with the issue]
AND (name = 'assign_filter' OR name = 'flextable_mod_assign_grading')
Usual caveats apply, make sure you know what you're doing and ideally do it on a test system or make a backup of your data first