Unable to Reinforce new Cavalry Factors

[Filmstudy]

This is an odd little error, but I am unable to reinforce with new cavalry factors to an empty corps in the Prussian homeland (Magdeburg). The factor exchange screen comes up, it just won't let me load the factors. I was able to get it to work after first swapping some infantry from the garrison, then returning to load the cavalry factors.

I sent files separately to monadman.