Snapshot managed backups won't work

I can't seem to be able to succesfully perform a managed backup using snapshot.
I need to create a staging website to test some changes to the live theme, I tired starting it manually and scheduled but they never come up in the WPMU dashboard.
The website has a lot of files (all below 40 MB), if I don't save files I can finally see an archive in the list of shapshots.
I have already changed the config file as suggested here
but I can't see any useful log in debug.log.

I have granted support access.

Can you advise how to be able to perform a managed backup?

Alternatively, could you suggest other backup plugins to create a staging website (a clone of the live website)?