i too was facing the same problem.
I resolved it by
-registering and logging into my account at my.avast.com on a computer preferably rather than a phone or tablet
-then go to the device section followed by the backup section
-go to settings and remove any and all ticked options and save
-log out on the computer and attempt the backup on your mobile device
I’m not sure if thats helpful it may have been pure luck for me but thought it may help