Answer to the problem?
My '93 400E has exactly the same symptoms as your 500E and was wondering if you found the answer to the problem. I suspect the passenger door assembly but not sure yet. Another quirk it also does is LOWERS the windows when locking and RAISES them when unlocking. It appears to be out of phase somewhere or like the 'battery terminals reversed' which I know it can't be!
Thanks for your insight.