Open Airport Utility and click on the picture of the Time Capsule
A smaller window will appear with information about the Time Capsule
Look for the LAN IP address and jot that down
Close AirPort Utility
Click anywhere on the open desktop of the Mac so that the Finder menus appear at the top of the screen
Click the Go Menu
Click Connect to Server
Enter afp:// and the LAN IP address of the Time Capsule. For example, if the LAN IP address is 10.0.1.8, you would enter the following:
afp://10.0.1.8
Click Connect
You will be asked for the Time Capsule admin password
If a connection can be established, then try running Migration Assistant again
If still no luck, unfortunately, I suspect that the backups are likely corrupted. You will need to contact Apple Support directly for additional assistance since I cannot duplicate the issue that you are reporting