Server is Cauldron, with no mods at all (my machine, so when the problem started, I just made a second port with the same software and dropped all the mods one by one to try to find the problem).
I can CONNECT to the server with no problems - It's the chest opening that crashes.
I also just realized - If I'm vanished - I can open it fine .. but obviously can't do anything vanished. I tried again removing all the plugins as well, and still have the same issue.
So basically a default, brand new install of server and client and the issue still occurs.
Edit: also, ty for the quick response.
Edit 2: Another .. small? noticing - I can open ender chests just fine. It's only trapped and regular vanilla chests. I've tried with both empty, and chests with items in it.