Amazing. This totally was the issue. I opened the mission file with 7-Zip and deleted all the "settigns" folders like DSMS, TAD, TGP, SADL, etc. Reloaded it on a server and now it works like a charm. Thank you very much Yurgon.
Thanks BIGNEWY for looking at this. I suppose this is still a bug (???) and not intended functionality but does not appear related to A-10C II.