Why is my Samsung Galaxy S II stuck in Airplane mode?

My Galalaxy S II phone is stuck in offline (flight) modus. The offline (flight) modus can not be switched off:

At start-up phone is already in flight modus (airplane symbol)

When pressing power button and choosing "offline-modus" the "offline modus becomes grey" and can not be reversed again. Phone is alwas in flight modus

Removing battery with device on for several minutes did not help.

Also removing battery with phone off does not help.

WLAN works fine but no calls can be placed (neither incoming nor outgoing) and no data up- or downloading over mobile phone network is possible

Anyone has a great idea how to fix this?