Mavproxy shows garbage APM: messages

I am seeing garbage messages in the mavproxy output.

 APM:  Y���9�h<eU

is there any way to trace down where that is coming from? Other messages make sense like

 APM: Disarming motors

search this forum. The same question was answered recently.

Edit: it is the first question on the FAQ: Frequently Asked Questions and Issues — MAVProxy documentation

Thanks. PEBCAK and RTFM