Hello Michel,
great work you did there. I have been using your MAVLink2HoTT for a while with a GR-24 receiver without problems.
I now did a new built up with a GR-12L and telemetry does not work 
I’m using an Arduino Pro Mini 16MHz board. Works on a GR-24 and GR-18, but not on the GR-12L. Other sensors work with the GR-12L.
I’m quite familiar with the HoTT Telemetry Protocol and also did a Text-Menu implementation for openXsensors (available at https://github.com/sternflyer/openXsensor-extended-HoTT). I checked the response of MAVLink2HoTT in Text- as well as in Binary Mode in a serial terminal. The Arduino responses with the correct data, but I believe the timing is incorrect. I know that especially the GR-12L is quite “picky” when it comes to timing.
Is there a possibility you can check the timing to ensure it meets the HoTT Protocol? It should be 5ms before sending data after a request and 2ms between each byte.
I’m quite happy to support you if you need help looking at timing in your code.
Thanks!
Kind regards
Niko