OK - so since Snapshot won't play with OneDrive, I upgraded my Google Drive to handle my 50GB+ backup for this site.
I went through all the setup instructions but on "Step 3: Authorise" I get a 'page not found' error. So I checked the setup according to the instructions. All good, hit "Authorise" again and, of course, get the same result.
So I think that maybe that's a quaint way of telling me it worked so I then set up and started the backup. Everything looked good for the first few minutes so I went and had a coffee while my SSD based/32GB of RAM server did the work. WRONG! I came back 20 minutes later to a Cloudflare 502 error message!
Now my server is fairly fast and I use the paid version of Cloudflare so there should not be an issue. But clearly there is since nothing arrived in my Drive account either.
Bandwidth is not the problem because I have about a gazillion TB or more. Can't be RAM because there's always at least 10gb in reserve and Snapshot only has access to 512mb.
I do run LiteSpeed but I sure hope that's not the issue.