[werk_ag]

It also seems to me just an initialization issue.
Just for test, you could try uncle_bob suggestion and in the user config options change the TestMode var to True

Code:

#define TestMode True

Alternatively, power on your TX unit, keep it 1 meter away from the RX unit, and power on the RX unit.

Initialization procedure may take from 30 seconds to around 2 minutes, please wait until the Com_LED, goes OFF.

After a successful initialization, put the VP1 switch at close position and start up Cumulus software. The RX clock will be set automatically.

Please, let us know the result.