[Imp]

Does look like an OOB error like you say.
Is it a generated campaign or a created one possibly with modified units?

Only other possibility I can think of is its getting tripped up due to new purchases I had that a couple of times but cannot remember the error.

I THINK its important to put the final battle save in WarCab
The one you get asked to save before selecting your next battle & making purchases directly after getting victory results.

Try starting a new Dossier from the save that gives the problem & see if you still get the same error.

If not I would assume that means the OOB is correct & it got confused with purchases or cross attaching.