Things have come to a good end
As my phone is out of warranty anyway, I used the recent holidays to get rid of the stock 2.3.4 and installed CM10.2 (nightly from Dec. 17th). As CM doesn't offer rsap, I also installed the rsap app to my S2. And now the app is working perfectly fine, not only with the Audi rsap adapters, but also with the premium phone of a 2013 VW Tiguan. Initially the phonebook access was buggy and only transferred about 10 phone numbers before hanging up, but the BT phonebook app also solved that issue.
The only very minor issue I only sometimes encounter (only) with the Audi adapter is an (automatic) reconnect some minutes after the initial connect - but this really can be neglected.
On my phone the combination of CM10.2 + the rsap app + the bohnebook app provides perfect results