Cannot use 2 telemetry connections

I am testing 2 telemetry connections:

1: LTE using UAVCAST, connected to Serial 1
2: Short range 100mW, 915Mhz, connected to serial 4

I power up the aircraft, start mission planner and wait for UDP to automatically connect.

I can then give commands to the pixhawk. So this is working. Then i right click on the top bar, choose connection options and select Com5, the baudrate and connect. I can now give commands to the pixhawk as well.

I can now switch between the 2 connections and receive the telemetry.

But… i cannot give commends through the UDP connection anymore. The screen freezes and after a while i get Error, no response from UAV.

Through the com5 (the second connection) is still working.

Even when i disconnect the 2nd telemetry, it still doesn’t work anymore.

Please help.