I'm having the same issue on my 540it. Tried to diagnose it using another 5 series at the yards today and the only thing I can reckon is that the latch mechanism is stuck and the actuator/door handle can't budge it. Same happens on my 325i once in awhile.
Trying to figure out if I need to perform surgery on my door card to get to the latch. (I have an exact spare if I need to... )
I'll post next week what I end up doing. Perhaps it may apply to our TI's.