There are occasionally issues but I doubt permissions is it.
Pondini has a few other suggestions.. did you read those.
There are some occasions when you actually have to delete the old backup and start over.. if you were using wireless for the backup this can happen as it corrupts somewhere.
Full factory reset of the TC..
New names for TC and wireless.
Then mount the disk in finder and do the verify as per pondini A5..
No luck delete the backup or erase the TC and start over.. this is occasionally the only way to get things restarted.