It appears manual backup is not always happening.
On my online development server with single site WP (non multisite) the manual backup works. I also have a multisite w. Buddypress on the same server, there the manual backup opens the front end of the multisite. However, on this multisite the scheduled backup works.
On another multisite on the same server, manual backup works.
On a clients website (single WP) today manual backup gives a white screen. No backup is made, so there is also no log file.
I haven't been able to discover the pattern here. Fact is manual backup can lead to white screen and nothing happening, and this is experienced by other users too. In other cases manual backup works fine.
I keep recommending this great plugin. Would be nice if this strange problem could be solved.