I've had a similar issue just today. I installed and configured MC on a new machine that I'm using as a file server. I set up the new PC to run MC in server mode. When trying to connect from the client I got the same message as you. However, I found that after a client reboot, if I go to Explorer and access one of the drives I have shared on the server, then start MC, it will connect and download the library. Another "fix" was to simply ping the server from the client before starting MC. Now I have a batch file in my StartUp folder that runs at logon and pings the server before anything else...works so far. Even with the preemptive ping, it seems to work better if the library I'm trying to load is one that I've manually entered (IP in Library Manager) vs. "Search for Media Servers".
From what I've found out so far I believe this is some kind of authentication and/or timing issue with Windows. I believe a very similar problem occurs when trying to reconnect mapped network drives at logon. Many people, myself included, get the "Could not reconnect all your network drives" message in the system tray, although if you look in Explorer the drive is there. I'm still investigating...
So you can try:
Reboot the client
Ping the server
Start MC
Search for media servers (repeat this step if it fails the first time) or In library manager add a library using the server IP in the "Location" field, "Name" can be what ever you want.