Hi! This is my first github issue so apologies if I don't format it/compose it right.
My friend and I are playing on All The Mods 10 with the mod Deeper and Darker, and though loot chests spawn in the ancient city, they are unopenable, and if destroyed instead of the standard loot box message warning, it just breaks and turns into a regular chest. We're on version 1.21.1 using neoforge if that helps!

Also, where possible, can you target one of the chests, type /data get block then space and it'll let you automatically fill in (just press tab) the coordinates you're looking at. If you could post a screenshot of that also.

Are you able to upload the server log that contains you attempting to open these containers, as well as the loot table loading? It's possible that the loot tables are corrupted somehow.

Alternately, it's possible that the block entities themselves have been corrupted hence no warning being made when breaking.

I will try to do so now!

-- This one is taken from my computer, and after I did the command, on my second time trying to open it, I was able to. My friend on the server also was able to once I had done that.
The same thing happened at a separate chest in a further away area for me (couldn't open it, did /data get block coordinates, was able to open it) but my friend wasn't. I made her an operator so she could try the command as well, though it didn't open for her after she did the command. She grabbed a screenshot of it for me.

For the server log, what log should I be looking for? I'm sorry if this is a silly question, I've never done more technical stuff with minecraft so I don't know what I'm looking for

That is strange, I'm not sure what else would be affecting it. It would honestly help me the most if I had a copy of the server log where you weren't able to open the chests. If you'd like, you can join my discord and send it to me via that, or you can upload it to https://gist.github.com