Q: problem connecting to the server “user-pc”.

is the user-PC server a Windows machine?  Follow these directions for Windows networking if that's the case:

Mac OS X: Sharing your files with non-Apple computers

What service are you using on the internet? Is it an FTP server?  Is it Facebook? Is it iCloud, Is it something else?

Are you setting the background with iPhoto or System Preferences?  Are you using an admin account on your computer?

I've been having an almost identical problem. I get that error-message if I to to System Preferences/Desktop, or if I try to view a slideshow in iPhoto - and I think very rarely at other times, but these two always give that error.

I've spoken to Applecare today who were rather baffled, and have escalated the problem - I'm awaiting a possible solution.

I have the same problem, but it happens when I connect my android phone to my mac. The server it is complaining is a retired NAS from Seagate I used as time capsule.

Same problem here, when I try to change my screensaver (was using last 12 months of iphoto pics before upgrading to ML).

very annoying, the share it is complaining about has been offline for over 6 months!  Did apple respond to this?