Hi.
The "invalid license" message that you are getting can be due to a few things, assuming that your copy of Ship Simulator is acually valid, of course:
(1)
The server you are connecting to is overloaded (doing too many other things as well as running SS MP Server). This can also happen if the owner of the server has tried changing some of the update timings or radii.
(2)
The MP server just needs re-booting
(3)
Your PC needs rebooting
(4)
Your internet connection or the Server internet connection is noisy or very slow.
There are probably other causes, but these seem to be the most likely.