Thank you mauried!
But as you can see in the example above the HC-12 understands the messages and also responds correctly to it but THEN starts these ERROR Messages.
I'm almost sure, that I haven't changed the serial format (8 data bits, 1 stop bit). But maybe this was the reason. I now have set the parameters back to SERIAL_8N1 (the default) and it works again.
Thank you very much!