Oh, gosh, Bluetooth in cars! Don't you just love it? It seems that VW group vehicles use the Remote SIM Access protocol (rsap) and that, in some devices (and, perplexingly, with some SIMs) can either cause a total communication failure and/or the problem you're experiencing.
What I think is happening is that the connection to the car is breaking that with the phone so that, when you disconnect, the phone thinks it's got a new SIM and tells you to upgrade your settings. No, you don't need to and you can ignore the message, but you won't be able to stop it happening.
Why this has suddenly started happening is a bit of a mystery, but it's probably due to a system upgrade, either in the car or the phone. If it's the latter, contact a VW dealer and ask them to make sure your car has the latest software. Sadly, though, car manufacturers aren't especially interested in which version of Bluetooth they fit and are often well behind what's in phones, resulting in all kinds of incompatibilities.
