Bad serial communication

Hi folks, one of my BPiM5 is running headless. For maintannance sometimes I use the serial uart port from another Pi. I establish the connection with cu and a 8N1 setting with 115200 baud. Basicly that works, but I have a lot of “yyyyy” characters on the screen. It doesn’t matter if I reduce the baudrate to 9600 baud. Any hint for me?

Thx & regards Berni