Hey Guys,.. got some news on this case
I talked to Apple and they gave me the following tips:
1. Backup your Iphone with Itunes
2. Shut down the Iphone, disconnect it from your PC
3. Hold down the Home Button and connect it to your PC. Hold the Button until Itunes states, that the Iphone needs recovery
4. Start the recovery process
This Process differs from the normal Recovery Process because it completley overwrites the Fimrware within the Iphone. The normal Recovery Process only sets the Iphone to factory defaults
My Iphone now works better than before, but not perfect.
Another thing apple said is to change your sim card to the newest available Version from your telephone provicder. Sometimes it happens, that sim cards get incompatible with new modem firmwares.
Good Luck to all of you,.. hope that we will get an update soon which fixes all errors..
Kind Regards
Wolfgang