Experiencing the same issue, 1.6.9.37. If my partner and I are both in the mines (or skull caverns) together, monsters will not move or attack, are invincible and just about all animations are broken. As an example, if you swing your weapon, there are white "wind" animations left stuck on screen where the blade traveled. If you break a rock, the chunks freeze in the air. Traveling to a new level of the mines will fix this, it's a bit of a crapshoot whether the level you're on will be broken or not as far as I can tell.
