final answer after investigation as we had the same problem with another project not containing any DNG image sequences.
and It was obviously... wait for it.... a stupid plugin... filmconvert nitrate, that was blocking the whole projects.
i don't know why i didn't start from there.
version 3.03 doesn't work with 17, version 3.06 does, and it's always a pleasure to dig to those plugin sites and licences.
have a good day all