but I hope you'll excuse the lack of DxDiag log due to obvious reasons, that is DX not involved.X3:Terran Conflict v1.4, nothing modified. Humble Merchant start, about 7 days in-game time.
I have successfully boarded in Priest Rings a Military Nemesis, using a Scabbard and a Blastclaw Prototype. Out of the 6 Terran marines ejected from the Scabbard, 3 made it and captured the ship. While waiting for an M5 to come with the jumpdrive for the Nemesis, what do I hear: Boarding ship, Halfway to the hull, Blowing the hatch and so on. Even an Aargh and Medic, man down! Result: out of the 3 marines who initially were aboard MY ship now, only 2 still standing. The "second boarding operation" with invisible personnel started about 3 minutes after the "first one" actually succeeded. Reload. PS I am certain that all 6 marines either boarded the ship, or that only 3 remained after some got blown up while floating toward it. I was staring at the Nemesis all the time after getting it captured... for the first time, and no marine landed on it. Anyway, something weird happened,like, a mutiny?
EDIT: I got even the dreaded, if unknown to some, audio message "One of your ships has been boarded"
There was something else at least as weird, as much as I take into account the level of training of the marines (4 stars in all fields, less stars for fighting): in the wake of the first capture, the Nemesis had 5 Ion Shards onboard, no shields. Subsequent boarding and captures left the ship with no weapons, no shields, only a few missiles. That is, the following 6 captures by reload until I left it at that (empty ship) and resigned the reloading.