I just tried to do a nightmare dungeon and ran into two foggy entrances that I couldn’t interact with or enter. Both said “join fight in progress,” even though I was running the dungeon solo 
Unfortunately, I had to abandon about 30 minutes of progress..
Did I do something wrong, or is this a known issue in 2026?
did you die to the boss? the boss needs a moment to reset and then the fog clears again.
I hadn’t reached any boss yet! and I hadn’t died or encountered any issues beforehand, it had been a completely clean run up to that point
hm.. yea was just making sure. You know, sometimes issues result from the most stupid things.
Never saw it like you describe it - but doesnt need to mean anything.
You probably walked into the final boss area and then backtracked right away.
Then the fight starts and the entrance is locked.
Happened to me also once.
Think it solved itself after a couple seconds.
Oh, I see! I did stick around and ran back and forth for a while (as I mentioned, there were two fog gates) and I even stood there for a minute or so while I was googling the issue.
I suppose it must have bugged out then 
It happened to me on the map with the bells where you fight it the biggest snail Queen
Confirming OP’s report here, I’ve just encountered the same thing. I’m playing by myself, had just started a Nightmare Dungeon (Wretched Delve) as part of a War Plan, and encountered this weird space with two doors into it and a health pool like it was supposed to be a boss room with two entrances, but both doors are fogged over with “Join Fight in Progress”. Clicking the door does nothing. I went back and cleared the other side channels, switched subareas in the dungeon, portaled to town and back, and nothing unlocked the room for me.
I have screenshots, if it’ll help.