I get this error once in a while.  For me, it almost always connects properly the second time I try to open the file.

Once a few months ago, after a power failure, I had to log into the server as an administrator and start the Connection Manager manually.

You can do this by clicking on the Sage 50 Connection Manager in the little tray (up triangle beside the clock at the right of your Task Bar), and if the button under the Connection Manager says Start Service, you should click it to start the service.

You can also get to the Sage 50 Connection Manager through the Start Menu under the Sage option and your program version.

If that doesn't work, there are several options in the Knowledgebase in an article related to this message.  Just search the Knowledgebase for Cannot open company file