It may be a temporary problem on the server side preventing you from removing the PC from your Circle. However, check your hub PC date and time, just to be sure it is correct. If it isn't, correct it, reboot, and try the removal process again.
If the problem persists, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve