Error L1 invalid socket

After we had a electrical problem, we restart the pc. Then we open the webpage and it appear that 4 users are online when the server if offline -_-. After it the users are trying to connect to the server and can't do it because they have this error:
Error L1 invalid socket Closedclient
may someone help me to fix it?

Zteam files
Win 7 with all updates.

It appears 4 users only because the conneciton status got stuck.
invalid socket is common issue to alot of people, if you google tihs error you will see 500 solutions, we can't repeat solutions all the time.