Hello, I’m experiencing the same issue as OP. As far as I can tell, variations of this bug go all the way back to 2016?
This is how it happened in my case:
I was doing the Crystal Cave 3F quest normally. I came to the crystal wall not knowing what it is yet (I didn’t save the miners at the top at this point) and it told me there’s a small hole in it.
So far everything is fine, so I killed bunch of Shamans in the cave around and gathered the gems they drop and returned to the wall (still not saving the miners)
I activated the wall and it shattered (as expected) but the it bugged out and the wall immediately appeared again after the animation was over (at this point I was stuck inside the wall, because I moved forward to go past it)
After a few seconds being stuck, I walked over the warp and it did teleport me to the Closed Area, but at this point I still didn’t save the miners, so I figured this place will stay opened and I’ll come back later. so I went back to Caves 3F (the wall was still there)
Thinking its probably just a graphical glich, or that maybe I’ll be able to re-do the quest to break it again, I continued Caves 3F normally and saved the miners
one of the miners now tells me how to break the wall (a bit too late, since I already did it)
so I went back to the wall just to find out its not possible to go past it, and the quest trigger no longer shows up, which means I can’t re-activate the quest either (the warp arrows are however clearly visible underneath the wall, I just can’t access it in any way)
So at this point I’m in exactly the same situation as OP and cannot progress this quest (the Mysterious Slate quest is active though)
I’m new to this game (literally started playing today) and I’m not particularly sure what to do, or who to talk to. There’s also no Ticket option in the “Support” section of the website, so I don’t know how to submit an official ticket either. I’m playing on the EU server, game running through Steam, Keyboard
Help would be greatly appreciated as I’m enjoying this game a lot…