I'm looking into this now and after some heavy investigation of server logs, I feel confident saying this is unfortunately an old issue that's resurfacing for a very small handful of players due to how we're doing the snowflake build area this year.
Good news is that because of what issue it is, your stuff is not actually lost, it's just not readable by the server at the moment... But, it's still readable by me! It will take me a bit to ensure that this won't happen to anyone else, but in doing that, it will also restore enderchest contents as well. Panda, if your issue was actually caused by this same problem, yours will be restored as well during this process.