Backup disk not available in Time Machine with upgrade install of Mountain Lion

I just upgraded my 2010 Macbook Air to Mountain Lion and everything seems to be working well, except for my Time Capsule backups. Ever since the upgrade, I have had an exclamation mark in the toolbar where the icon for Time Machine sits with the sign "Backup Delayed." When I choose "Backup Now," I get the following message:

"Time Machine couldn't complete the back up to xxxxxxx. The backup disk is not available."

What is really odd, however, is when I actually go into Time Machine, my pre-Mountain Lion backups seem to be there, and I can mouse over the horizontal timeline with the old backups, though interestingly, they are all listed in a deep shade of purple, not the usual white. Still it seems that I can go back to an older date and see the contents of the drive at that time.

Im puzzled, because if the backup disk was simply not being recognized, then I would not have any history at all. (Incidentally, on the Time Machine toggle setup page, it lists 'Oldest backup: None' & 'Latest backup: None' which also doesnt square with the behavior actually in Time Machine.) In any case, the machine will not make any new backups and is stuck in this permanently delayed status for the last two days.

I would really appreciate any thoughts.....