I don't know what happened, but for some reason Google backup stopped working.
I didn't change any settings.
When you try to make a backup, the following message will appear: Failed to complete the backup, please try again later.
I tried over WiFi.
Tried via Mobile Internet.
Tried when the smartphone is charging.
(when the smartphone is charging) there is also no backup.
Used on Google drive 10.91 GB out of 15 GB.