The "Olita" region in Russia (right by Kovno) seems to have a glitch. If German units enter that location, they have to stop and siege a fort, but there is no fort shown on the map. The siege mechanic also doesn't seem to work there. I had some siege arty there and it wouldn't let me bombard the "phantom" fort.
Fwiw, in real life I think there was a fort at Olita, so I think it just needs to be a town with a fort. Hopefully that will clear things up.
Doug