by Ken Task.
Is DB host localhost? Is your instance on shared hosting?
Both moosh and the admin/cli/backup.php script use config.php info - DB user. Using mysql client, logged on as user for DB seen in config.php,
show grants;
If you can, change DB user/password in config.php to superuser credentials then execute commands.
After success, you can change back DB user/password.
'SoS', Ken