Minecraft Server player data is gone (new UUID)?

Hi, I've had my own MCS for months and I recently restarted the server PC (Win10 in the basement) and loaded a backup. But then there were problems but then I fixed them. But when I join the server again I'm (and everyone else from the server too) for the server like new players. I had to give myself a new op, etc.

All spier related data was new. When I looked in the ops.json, I was listed there twice with two different UUIDs!
Can someone help? Have I and everyone else lost all of our inventories, advantsments etc?

Possibly helpful information:

The server PC had to load 4-6 Windows updates before the problem occurred (but is now up to date)
I tried to load backups you but the plazer data were always wrong.
I think my server.properties was reset or something. Some settings were on Deafault.
The other servers in my bungee network do not have that problem and all player-related data is correct.
All my servers are 1.16.4 spigot servers with plugins and datapacks.

I have different UUIDs on different servers. Should that be so? I thought every player has an unchangeable UUID (see also picture. These are the ingame nbt UUIDs).
From the not problem server:

From the problem server:

On the problem server I have the UUID: "037e4ced-3314-32c9-a379-4be6cf401297" which according to (https://de.namemc.com/profile/037e4ced-3314-32c9-a379-4be6cf401297?q=037e4ced-3314-32c9-a379-4be6cf401297) does not exist. And "886c47a0-4546-4526-9381-0ab6ff4c3c7e" is the one from the other server that is giving out and mine is (https://de.namemc.com/profile/Leelux.1?q=886c47a0-4546-4526-9381-0ab6ff4c3c7e).
I can use all commands even though I have NO OP, even if I have just given myself an OP.

My theory is that the server doesn't communicate properly with the Minecraft server and it feels like I'm just another player for him.

You said that your server.properties may have been reset. Check whether the online mode is false when the server is integrated in the BungeeCord network.

I also had a problem with the UUID like that, it was because of the online mode.

Do you use MySQL? If so, did you connect the server with the permission plugin to the network database?

So the online mode was false and I just made a new server with a backup for and nothing has changed. But it must somehow be due to the server settings. But I don't know what. But thank you for looking at my problem! 👨🔧