Photos app wants to upload my library to iCloud once again
Had same problem, steps to start syncing again:


open system preferences
iCloud
control...
locate your Media data and click delete
it will be marked as deleted and available for download during 30 days, free space will show all of your space
go to Photos
Settings...
click [x] iCloud
now it will turn on
go back to System preferences
iCloud
Control...
locate your Media data and click Cancel Delete
be happy
let me know if that helped ;)