Connection lost

i run Mineos in a proxmox vm.
on my Server i run a modpack (all the mods 6)
i connect over lan and 2 friends over internet
we all 3 get a error after random time (5min-2h) but all on the same time:
connection lost
internal exception: Eine vorhandene Verbindung wurde vom Remotehost geschlossen.

a few seconds before the error the interface say: waiting for server…

the server is still running and shows no logs just
[19Apr2021 14:23:41.160] [Server thread/INFO] []: DangerDragon3295 lost connection: Disconnected
[19Apr2021 14:23:41.160] [Server thread/INFO] [net.minecraft.server.dedicated.DedicatedServer/]: DangerDragon3295 left the game

Sorry for my bad english and greetings from Germany

Are all 3 connections using the external IP address? Any other devices lose connection at your location?

I use the internal ip of the server (
For the other guys I use dyndns from Strato over my Fritzbox.
Other devices work normal.