Modded Server gets stuck on "[register]: refreshing ip addresses"

I'm writing a mod and it was working fine until I wanted to add in a custom entity that inherits from projectile and gets created by a tile of the map's game-layer (the one on the right of the rifle).
Now when I want to start the server it just gets stuck  on "[register]: refreshing ip addresses" without any error and I can't find it on the serverlist either.

If I start with a map that hasn't got the entity on it, it starts without problems. When I then change the map to one which has the entity in it, the client disconnects from the server. The server is still running in the terminal, but is again stuck. Maybe it got into a endless-loop or something.

I used the search function, but only found something for an old version, and the solution there isn't applicable anymore as far as I can tell.

Can anybody point me in the right direction where to look for the problem?