I tried the steps you outlined and reached the factory reset stage. Also running iPhone X and the latest ios versions and viber. Still I could not delete the viber backup data.
I tried using the iCloud drive on Mac and did the manage storage steps in the System Preferences/iCloud similar to the iPhone. Still I could not delete, with the message that it could not be deleted, and try again later. I have tried several times but still no luck.
Have you had any success lately?
I did some exploring in other forums on how to access iCloud drive files on the Mac. I am not strong in the use of terminal commands and thus followed but changing some parameters or words.
Using:
cd ~/Library/Mobile\ Documents/
find *viber*
I saw that a lot of the files occupying my backup were jpg and mp4 files. And that these were located (locally or remote I am not sure how to interpret):
/Users/Username/Library/Mobile Documents/iCloud~com~viber/backupname
Listing the contents in the backup directory, I see these:
. .. ..primary.viber.icloud Media
with the jpg files in Media such as:
iCloud~com~viber/backupname=/Media/.0-02-06-32799cf3011bfe745072e9710c7d0accfaf4ab9982bf478bc7757753cb265c8b.jpg.icloud
while I can drill down to the Media subfolder and list (ls -a) the contents, I don't know if I can (not sure what command to use), or should I even try delete these jpg and mp4 files via terminal commands.
That's as far as I've gone. I hope to hear how best to proceed with this.