LR does not let me synch my mobile collection although I have done it before. I get a message now that says "Sync has been turned off. This appears to be a backup catalog which maybe out of date with LR mobile. I have done the "Delete All Data" step several times now but keep getting this message. Any suggestions/workarounds I can try? I am using my main catalog - not a backup.
Did you run in to a Lr Desktop crash which caused the issue? Could you try to re-sign-in via Lr Desktop help menu and let me know if that helps? - Guido