OK, once again.
I have no clue WHAT happened, but I now can't move my land units onto ships whatsoever.
I won't upload a save HERE, because it happened like 80 turns earlier in-game, and I didn't want to risk a MAF to also open Opera.
I totally couldn't board an Outrigger with a "buffed" soldier (a General and a few Size-Ups).
But soon-ish (in-game-wise, because I'm now pausing my gaming until tomorrow) I will most hopefully finally unlock the Galley, which is clearly a transport ship.
So, if (or more like WHEN) it fails to work as intended - I shall post a save as well.
But I see this being a recurring error, so MAYBE you should look it up in the files first.
I would really appreciate that, thank you.