Very concerned - Failed to backup file or folder error

Windows 10.  TI2018 with latest update.

I had set up an incremental backup selecting a number of folders, including C:\Users.  During the backup, I got two errors, which I had to ignore for the backup to proceed to completion.  The error was:

Failed to backup file or folder "C:\Users\***\".

I thought this must have been due to an open file.  Why would it not be possible to backup a folder?

My assumption was that everything was ok.

However, I then created a folder and a file in the above folder and ran an incremental.  Got the errors again.  When attempting to restore, the new files where not available to select.

I also noticed that the "Saved Games" folder under my user profile folder had never been backed up.  What else might be missing?!

My user account is a local user account with admin rights.

Needless to say, this is extremely concerning.  I am now in a situation where I have no confidence that my backups are valid.

I'm sorry to say, having used TI 11 for many years, during which it never failed me; my first impressions of this product are highly disappointing.  E.g. there is no ability to access full logs from the interface(!), and it is not possible to delete specific backup instances(!).  That these features are missing is beyond belief.  Now my backups are in doubt,  I wonder whether I should even carry on using this product.   Hopefully I'm missing something, and it's just user error.

From the log:

10/06/2018 11:03:25: 00 9716 E0004000C: Failed to back up file or folder 'C:\Users\***\'. It may be blocked by another application.
The file is read-only. (0x4000C)
function = "OpenFileW",
path = "\\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy3\Users\***\OneDrive",
$module = "ti_demon_vs_11530"
winnt_dir::iterator::GetInodeInfo:
c:\bs_hudson\workspace\332\core\file\windows\winnt_dir.cpp(1,474)
The media is write protected (0xFFF0)
code = 80070013),
$module = "ti_demon_vs_11530"
Common::Error::AddWindowsError:
c:\bs_hudson\workspace\332\core\common\error.cpp(307) : None

(I have replaced my user name with *** above).

I have created a job to backup the entire disk instead, and this does not produce the same error.  Hopefully these are to be relied on.  I will be checking.