Rabbiofberlin,
I went through the teshuvah of the Bais Din that was “mevatel” Rabbi Druckman’s conversions (I think it was posted in another thread, DY?) and they didn’t say that it was because there was no kabolas mitzvos, or because they knew that the individuals weren’t sincere (that would be devarim sh’b’lev). Their reasoning was that there was no kosher Bais Din at the geirus.