I've got a Sony Xperia Z. A couple of weeks ago a Google Photos notification came up saying that 1790+ photos have not been backed up to check settings and update storage.
The settings hadn't been changed at all and I still have 2gig available storage.
I could manually sync photos by selecting them in Google Photos, but the automatic syncing no longer worked.
Around this time the battery began to drain a lot faster than usual and the UI got very laggy. I thought that as the phone is over 3 years old the Operating System might need a refresh. So I installed Cyanogenmod 12 (yuga) on the phone and reinstalled my apps.
I have repeatedly deleted the cache/app data on Google Photos.
Going into Google Photos > Assistant it shows the message:
Backing up photos 1823 items left
Even left plugged in overnight no sync progress was made. The battery is draining faster than usual (up to 10% an hour during normal usage) and Google Photos is showing as using the most battery power (even though it appears to be doing nothing).
Any ideas about what is going on and how I can fix this issue? Ideally I'd like all my photos synced again.