Hi Farin,
Thought this was fixed in 4.1.x, but just played another game with a colleague in the office (so able to compare screens). Whenever one of us plays a barn or pig, the opponent sees them play a pig or barn respectively (i.e. I place a pig, they see a barn being placed, usually illegally). This obviously then throws the two games out of sync and into NullPointerException.
Having saved the game, and deleting the previous moves, we could then complete the game as long as neither of us played a barn or pig. As soon as we did, the same error occurred.
Save file available if required.
Linkback: https://www.carcassonnecentral.com/community/index.php?topic=3808.0