After working fine for months, my wifi Time Capsule backups fail, returning a "back-up disk not available" message. Time Capsule light is green, Airport Utility reveals no problems. What do I do? Thanks.
Are you connected to the 5ghz wifi or the normal wifi ?
I had the exact same problem you describe, Airport Utility showed nothing wrong, green lights across the board, but when trying to do a Time Machine backup, it wouldn't be able to find the disk.
Additionally, I was also unable to access the Time Capsule's disk through Finder. It shows up alright, but I could not connect.
Manually switching wifi network to the non-5ghz one solved both the backup and Finder problems.
From Tesserax's answer from June 2018 on the Apple Communities:
Use a 16-character or less base station name for the TC. Also do not include special characters or spaces in the name.
This solved the "Backup Disk Not Available" for me in 2020 on macOS Catalina with an Airport Time Capsule version 7.9.1.