Snapshot failed backup files

I have tested Snapshot on several sites, both Managed Backup and local failed when backing up files.

It's either "An unknown response returned from Snapshot backup attempt. Aborting. Double check Snapshot settings.", "<empty response>" or it fails with a 404 page inside backup tab.

Tried increasing WordPress memory, PHP max_execution_time and using various Snapshot flags in wp-config.php but still failing at files backup.