What it says on the tin- when a player attempts to retrieve items from an old grave, their entire inventory is wiped, including worn items, and the console is spammed with the following message:
https://mclo.gs/WYPz159

Minecraft Fabric 1.20.1
YIGD v2.0.0

When you say "old grave", what exactly do you mean by that? One that has existed for a long time, gone through a minor update, a major update, or what exactly do you mean?

I think I can see what has happened. I presume some mod has been added before the grave generated, and now this grave does not contain any items in an inventory container expected to be added by this other mod. I tried to make sure this error would not happen in this update, the thing is that it seemed to work sort of fine before the update, but not anymore. If this is the case I know how to fix it though. Perhaps I'll try to release an update later today, but could as well be a few days until I do so.

Updating it on the server should be sufficient, unless I make any changes to the client, which is as of now not planned. I'll let you know here when I update and in case I make changes to the client

A few days turned into a few weeks. Part of the reason why it took the time it took is because I had my exams. I can't remember if I actually fixed the issue or not. I'm having a hard time seeing a world where I didn't, but in case I forgot, let me know.
I did change some item registration stuff in this new version, which usually means clients need to update, but in this case I actually don't know, as I'm not aware if minecraft will sync disabled blocks/items/other features. (And disabled items is what I'm using instead of conditional registration). You're welcome to try and see if it's possible, but just count on the possibility it might not work.
Thank you for reporting the issue though! If there's anything else you find, let me know!