There could be a number of explanations for this issue.
Did you enter the correct server address?
It could be that there's a typo in the address that you entered. Or it could be that the port number that you entered is incorrect. Ask your server administrator for the exact server address including port number.
Depending on which Acrolinx version you’re using, you may see slightly different behavior.
Acrolinx Version 1.5 Windows Word sidebar edition
When you click "Connect" to test the connection with your server, Acrolinx may show a status message "The server is online and everything loads properly." If you select Show Sidebar in your editor, the sidebar window is empty. The plug-in is trying to load a sidebar but doesn’t have a connection to the server.
Ask your server administrator for the exact server address including port number.
Acrolinx 1.5 SR1 Windows Word sidebar edition
We removed the Connect button since Acrolinx no longer tests the connection. Make sure that you don't have a typo in the server address. Ask your server administrator for the exact server address including port number.
Do you have a proxy that may be blocking the communication between the sidebar and the Acrolinx server?
If you're not sure, you might need to check with your IT department.