Is it possible that the Share on the desktop PC has changed to require a user name and password and the other computers in the circle are not using a login ID and password known to the PC hosting the share? I'm not using the feature in 2.0 for centralized backup, but I know that in my home network, if a share requires a User Name/Password, and I am logged into another PC with a different ID/PW than the one I'm logged into where the Share is located, I am prompted for ID/PW to access it.
Another thought is that it could be related to the problems we experienced with Circles functionality that was fixed yesterday, a day before your post, actually. During the problem period, many people had problems with Hub PCs reporting the inability to reach the monitored PCs, though they were on the same LAN and in green status.
-steve
