Through a crazy series of missteps, I managed to break the game at the Brood of Blood/Flesh/Dread mausoleum. Here's how...
1) Had NPC Kythaela "Misty Step" to the back room, after dropping the column (none of my other PCs could access that back room tho they had long jump, high athletics and even fly).
2) Kythaela lured each of the Broods out onto the fallen column and my PCs killed them with ranged attacks - she however was now stuck, since she was out of spells (LOL). No more Misty Step for her!
3) Leaving her behind, my PCs went to the long rest area (in hopes it might also give the NPC some options)
4) Immediately upon activating the long rest, a fight broke out back at the mausoleum between Kythaela and at least four Broods. The screen zoomed to Kythaela and the party interface disappeared. The log however remained active and showed 'lots' of initiative rolls for the Broods (many more than the 4 that were showing up in the mausoleum) - and even initiative rolls for my PCs back at their long rest.
5) With the interface gone, my only option was to Esc. Save was greyed out, but I was able to grab a screen shot and quit the game. Reloading from the save fixed the bugged UI and entered combat normally, however with Kythaela still stranded (and spell-less) and the party back at the long rest area (with a long series of turns dashing before they could get back into the battle).