Well, I was hoping for a networking guru to leap in with a magical answer.
Even asked one directly; no help.
So, all I can suggest is the usual "grab bag" -- hopefully one will do it:
• Re-select the TC via TM Preferences.
• If that doesn't help, do a "full reset" of Time Machine, per #A4 in Time Machine - Troubleshooting.
• Reset the TC. I think a "hard reset" is what you want. See http://support.apple.com/kb/ht3728
• If stil no help, power your Mac and Time Capsule down. Leave them off for several minutes. Power the TC back up first. Wait several minutes, then power up your Mac.