"Backup Disk Not Available", Time Machine can't find Time Capsule any more
I had a similar effect a few days ago. I was able to select the disk (see your last printscreen) again and from then on it worked flawlessly... Can you select your disk again?
I also had a similar problem backing up my data to external drive. But I figured it out by opening Disk Utility app on Mac and check what's wrong with my storage. And I saw that backup disk hasn't mounted. I tried to mount it and it was successful. But before mounting I needed to enter my password because it was encrypted.