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GDrive - Backup failed, Invalid status code: 308 was a previous case suggesting it’s a mono bug although Google’s possibly unusual use of the 308 as Resume Incomplete is arguably part of it.
WebException.Response is null under Mono, not null under .net (archived in Wayback Machine)
What’s the deal with HTTP status code 308?
Duplicati code which would handle 308 better (I hope) is shown there, but mono’s reject might be:
Possibly one of the Duplicati devs will have some thoughts on whether or not this can be helped.
If not, I guess check your network connections, maybe restart things, maybe reduce volume size.
1 GB is quite a bit over the 50 MB default. If you suffer a temporary network issue, reducing size
possibly will allow upload to finish (at current, watch About → Show log → Live → Retry to see if
retries are happening currently), and if you can raise it later, compact should repackage to larger.