This bug had suposelly been fixed but was brought back in a recent patch.
"03/06/20
Fixed the campaign map defender siege engine positions for the level 2 version of Medeni castle. This should resolve crashing for cases where the player initiates a siege against a level 2 version of the castle. "
Level 1 and 3 are fine (well lvl 3 at least didn't try lvl 1). But the game crashes on attacking lvl 2 Medeni castle on 1.5.3.
On my current campaign I did got crashes on any atempt to siege Medeni at lvl 2. Then I found this post and the patch notes and tryed to siege it later on after it had upgraded to lvl 3 and it worked fine.