I can't see anything in the code you posted that looks relevant. Are you getting a error message like "MOVETYPE_PUSH with a non bsp model" or are you just seeing a monster that's frozen solid and invulnerable?

If it's the latter, it can be helpful to get a snapshot of the entity data of the frozen monster. One way to do that without much fuss is to save the game once you've reproduced the error, then open the save file in a text editor. Then after a bit of searching for the classname you should find the stuck entity. Easier to do in a test map with a small number of monsters!