I have been from time to time getting "cannot back up to Creative cloud" with no reason why. I have searched and searched and cannot find out why. I have starting backing up to iCloud as it works every time. The links to art etc., in the Creative Cloud, work fine, just not the main file, sometimes it does and others it doesn't. If I rename it then it seems to go, but I can't do this all the time or I will have so many duplicate files to search.
Sorry to hear about your difficulties. What are you attempting to backup when this error occurs? What program is creating the main file you make reference to? Would it be possible to capture a most a screen shot of the error next time it happens?