I had this issue with a similar user. We could not delete her flagged items because of the same error you got with the exception of line 950.
It turned out that she was on another list in which she was retaining all emails. Once we removed her from that list (temporarily) we were able to then delete the folder containing her flagged items (tasks). After doing so we had the user restart Outlook and it successfully repopulated the tasks with not only her old tasks but the ones she was flagging while we were trying to figure out the issue.
Hope this helps!
It turned out that she was on another list in which she was retaining all emails. Once we removed her from that list (temporarily) we were able to then delete the folder containing her flagged items (tasks). After doing so we had the user restart Outlook and it successfully repopulated the tasks with not only her old tasks but the ones she was flagging while we were trying to figure out the issue.
Hope this helps!