I hosted a Nitrado Minecraft Forge Server for 10 slots on the 1.8.9. However, I always have the problem when joining that there after about 5 seconds "Disconnected" is. What can I do the server is re-hosted and there are no additional mods in the game. Already 1000x the server reset.
Please ask Nitrado Support.
If that does not help, go to a clever hoster.
What does the log say during the crash?