He says it starts to load, so obviously he can connect.
Anyways, it could be a bad add-on.
On whatever computer he is using, go into the Blockland folder, goto Config, then Server, and open ADD_ON_LIST.cs
It will give you two options, pick the bottom one.
A window should pop up with a list of programs, select Notepad.
Once notepad opens up with the file, copy all the text, and paste it here.