I've run into that one before and It can be caused by two things. You may have multiplayer details saved from another server in your:
My Games\Freelancer\Accts\MultiPlayer
..folder. But in that case you wouldn't be able to join either.
The other one is that you have clients who have a modified new_character.ini file. I don't know why it does this as it is supposed to be server side, but either way having a startup xml interface which changes details in new_character means tha they will have trouble joining.
The fix is to remove the new_character changes and put any sever based xml interface as a stand alone mini-mod. You want to have clients using the basic SDK for the game to allow them to join properly.
You could set the new_character to have different details if you wanted to, as long as the details are the same on the server and the client.