For me, the fix to this would have to be entering in a "port" number.
When Blockland starts up, look in the console, you should see something like this.

Remember the number you see there, it changes every time you start Blockland.
Now when you start the server, go under advanced and find something that says "Port" and make sure to enter the number you saw.

I have been doing this to host servers, and it has never failed for me once.