Time Capsule suddenly full

I have backed up my iMac to a Time Capsule using Time Machine for half a dozen years. This has worked flawlessly for the last couple of years and indeed transferred all my data when I bought the new iMac about a year ago. There have been times when TM told me the TC was getting too full and it quietly got rid of the old backups. Suddenly I have received the message "backup failed…" and I note that Time Machine no longer has a year's worth of backups - all that's left is a couple of backups from yesterday.

This is my question: has this occurred because I changed the name of my Photos library? and does Time Machine think that this is a new 100GB file which needs to be backed up?

I should explain: I have been using iPhoto for ages — I admired the Photos app on iOS — when Yosemite arrived I started using Photos on the iMac — then I was dismayed to find that I could not add Locations to photos so I reverted to using iPhoto. Now El Capitan offers a new version of Photos which will add Locations but I was concerned that the new app would not pick up all the new photos which had been added to the iPhoto library after it had been designated as the System Photo Library. (Sorry, dear reader, but this long off-topic story is to give background on why I renamed the file).

I spoke to AppleCare and, in helping me with this problem, the agent asked me to rename the Photos library.

Immediately after this was done I received the "backup failed" message so it looks like cause-and-effect.

What should I do? revert the names of the libraries to their original names? destroy my backups on the Time Capsule and start again? I'm pretty nervous about doing both of those things so it would be nice to know if there is a less drastic solution. I have consulted Maestro Pondini's web advice but I couldn't find exactly this situation.