I had similar problems when going from V25 to V26. Lost various menu settings. The only solution was to visit the dealer, who reprogrammed the Nav free of charge - took about 5 minutes. Everything is now fine.

Issue was resolved after visit to the dealer, and a new BT/ASsist TCU module - #84-10-6-978-529 to be exact, which I beleive is a brand new model.
However, now I have an issue with my v710 phone book loading... uhg! :mad:

Sorry to dread up this old thread, but I've now got exactly the same problem .... and I wasn't even updating the NAV O/S. It just started doing it. I also get a message that BMW Assist is not active (even though the subscription is still ok). Guess a trip to the dealer is in order ;-)

That's probably the way to go... hopefully you're still under warranty/maintenance!