Welcome to my world. I had this problem 1st time on December when I return home and found out I can't go reverse.
So I sent my car to some BMW raring team. They stripped tramission and found nothing. When they put it together the reverse gear was working again suddenly so they gave me my car back.
Sadly, the same problem has occurred again 2 weeks ago... they have some theories like:
1 - the "fork" which is moving with gears is bent
2 - maybe something is wrong with gear lever
3 - ...
Here I make some video for those mechanics.
I am speaking in my language so turn on subtitles if you want to understand.