Damage immune monsters

Unfortunately, I haven't been able to reproduce this reliably. It seems to happen at random. Both times it happened to me it was in the prison in act 1, unsure if it was the upper or lower or both.

I was using frozen pulse. Lifesprigs, Thousand Ribbons, Crown of Throns, Lochtonial Caress, Wanderlust and Stone of Lazhwar.

Both times it happened I was in a doorway.

The first time, moving out of the doorway enabled my spell to hit and do damage again.

The second time, moving into the room the monsters were in had no effect. I tried at many different angles, and with a normal attack, none of them seemed to hit. Finally, without any changes that I noticed, they were able to be hit again.

I was using predictive network mode, which is a setting I recently changed. I don't know if this is related.

This is the possible disadvantage of the predictive mode, that your client can go out of sync with the server, so on server side your character is at another position (e.g. before a doorway), s.a. Networking mode.
You can try to use the chat command "/oos" to get in-sync.