I note that other users are also receiving these warnings from Google Drive. So, I just want to provide my conclusion to this issue. As stated in my last reply:
I have previously had a bad experience using OneDrive and the Cloudbacko backup software (I seem to recall it was said on the former Cloudbacko forums that OneDrive imposed a throttle?), so I switched Cloudbacko to use an SFTP server and this reduced the errors to only be occasional.
Following my research quoted above regarding Google Drive, I feared that I was experiencing the same limitations with Google Drive and because I continued to get these warnings, I decided to ditch Google Drive in favour of Microsoft Azure. I’m pleased to say that I haven’t had any problems with Duplicati since doing this and I have been running on Azure for about a year now. Before switching to Azure, I upgraded my internet connection to a bigger upstream, in case it was influencing the problem, but it didn’t help.
Furthermore, I have also switched Cloudbacko over to Azure, from the SFTP server that I was previously using, and I have had no further errors with Cloudbacko either - it runs very smoothly too.
I have since decided to setup a further 2x PCs with Duplicati; one also backs up to Microsoft Azure and another backs up to the free Backblaze B2 storage (but the data on that PC is small and barely changes) - again, no issues.
So all I can say is that since using backend storage that is made for the task, everything runs smoothly and quickly with no errors. So I have concluded that a lot of the problems come down to unreliable backend storage rather than the software being used.