(Server Self-Hosting) Weird Timeout after some time.

I wanted to ask if someone could help me figure out this problem. I host a Minecrafter server for my friends and me. They can connect normally through the Ip and already forwarded Port (25565). But than after like 30 min to 1 hour if they disconnect, they can't reconnect unit i restart the server. Players can still play without any problems, if they don't disconnect. No one gets kicked. The error the person gets when the try to reconnect is just a normal "Time Out". Any ideas?