I get the very same error since a couple of weeks. For some reason, PFPX now uses the aircraft's registration instead of the callsign used for the flight, and leaves the registration empty in the remarks section.. I got this error for the first time yesterday, while planning a new flight from my flight schedule. Plane is selected, flight has a (alphanumeric) callsign assigned to it.
So just to clarify:
Stephen, in your screenshot you can see that in the first line, a 'normal' callsign is submitted. (ESA7903), and the aircrafts' registration is placed in the remarks' section. (REG/ECETK). This is how it's always been for me. But for some reason, as of yesterday, I get the same outcome as Torben; it replaces the callsign with the aircrafts' registration, and leaves that blank in the remarks' section.
(FPL-ECETK-IS while it should've started with (FPL-ESA7903-IS.