Error message on minecraft server?

I'm admin at a server and when our people come to the server this message appears after 2 minutes or so and you can never play normally that you are always kicked out but why? What can you do as a server owner so that you can play normally again without this window? And the server is running well

I found this at Nitrado:

I deleted the Minecraft folder in the% appdata% and then turned on the launcher that has to download Minecraft again.

Then before I logged on to the server again, I also switched off my firewall.

Lo and behold, I was no longer kicked from the server. I switched the firewall on again and was still not kicked by the server, so it is probably not even necessary to turn the firewall off.

Therefore, simply delete your Minecraft folder in% appdata% and download it again. But be careful, all server data and login data will be deleted!

What do you mean with server data deleted should you download the best in a backup or something?

This is definitely a problem for the clients. You can't do much as an owner. You can try researching 12345121782828.

The saved servers are deleted and the created worlds.

Can you upload it again with a backup?

So you meant the host with clients or

You can copy the worlds out (folder "Saves") and you should also save the resource packs.

Ok and then upload again and you have everything again?

Clients are clients. So the computers / the game on the computers of the users who connect to the server.

Exactly. So copy out and delete the folder. And then let's see if it helps ^^