Robin2:
I don't have any HC12s so I can't help with that part. However if it was my project I would check that the communication works at the baud rate you want when the two Arduinos are connected by wires rather than using the HC12s....R
The issue was an incorrect transmitting Arduino baud rate setting. Somehow a bug set the baud rate forever at 115200, the HC12 automatically compensates for that but not below 4800. I solved the bug in the Pro Mini, issue gone.
I found out after connecting a scope to the Pro Mini TX.
Very usefull learning curve, and thanks for the help here!