Just another quick update.
We are still investigating these errors, most of which appear to be tied to One Drive, but it seems not necessarily always tied to One Drive. The results from the internal investigation indicates that the problem is a bug introduced by Microsoft. Just to give a simplified overview, when using VSS we aren't ever actually backing up your files, we backup Shadowcopies of your files. In the case where we run into problems, after we access the bundle of Shadowcopies that Microsoft is preparing for us (after using standard VSS calls), we encounter write access violations. So VSS prepares everything, gives us this blob of data, and then says we aren't allowed to access it. What's also strange is we aren't actually writing during the scan, we are just reading the data. We confirmed that this isn't just bad error mapping on our part, we are actually receiving write access violations.
We are implementing some changes that will be a part of the next release (v15.1) which should allow the backups to continue beyond this error. This is currently scheduled for release later this week. We will continue to work with Microsoft to address the issue further, whether that means changes on our end, or a patch on their end.
We really appreciate the feedback from this forum thread, as well as the all of the information submitted to us via web tickets.