Took a bit longer, but here are my experiences: I did a flight from EGPH to EDFH, as said. At first, everything went well. VoxATC did assign me a taxi route, and did guide me well.
When entering German airspace and switching to Langen, some odd things happened, as also mentioned here before: AI traffic was continuously reading back and after a while, ATC was continously acknowledging, like:
"Climbing FL380" "Climbing FL380" "Climbing FL380"...."Descend FL200" "Descend FL200" "Descend FL200".
This was annoying, but then, when getting closer to my T/D, Langen started like "Climb to FL420". My cruise altitude was 390. I started with my descend and approach, disabled and re-enabled VoxATC, but Langen kept saying "Climb to FL420". Even after landing, EDFH Ground kept saying I should climp to 420.
So, as I disabled migration tool before flying, I will try Mad_Mac's suggestions and keep it running throughout the flight.