I think it's likely the crash with the incoming fix based on your playstyle (at least I hope!). I'll ping this topic when the next patch is up with the crash fix and would love to hear if it fixes it for you. Should either be tomorrow or Thursday.
That error message does not sound very helpful, so no need to send it. It's supposed to drop a stack trace, but I may have set that up wrong. I'll look into it.
Glad you're enjoying the core systems! The balance is definitely off now. It has teetered between incredibly hard and incredibly easy, but it'll get there! More content in the future will help with difficulty, like enemies that can target your back line or remove block. I'll probably make the demon hunter parties spawn more quickly in the short term to apply a bit more pressure.