Guild Wars 2

I’ve tried twice to abort the instance by logging out, shutting down the client and logging back in, but without success; the Nightmare Court enemies always appear with invulnerability.

Upon dying during the Instance and restarting at a checkpoint, the Nightmare Court enemies are no longer invulnerable and can be damaged normally.

My wife and I had this same problem. We didn’t die, but we did try leaving and restarting it once.
The first time through, we killed everything but two nightmare hounds at the end (the ones earlier in the instance were still killable). The second time, we had the same problem, but it also extended to several spiders.

We finally resolved the issue (for us) by leading them away from the area where they spawned. After leaving that circle, they became vulnerable again and our progress continued.

I had the same problem, and also fixed it by leading them away. I ran down the hill. Once they followed me away from their spawn area they became vulnerable. I think it has to do with a part of the combat system that makes mobs invulnerable if they can’t reach you, to prevent sniping from safety, I guess. But these mobs could definitely reach me. But that might be part of the bug.