"File not present in the snapshot" error but the file is there

I'm getting the "File xxx is not present in the snapshot" warning during backup, but I've verified that the folder and files being referenced in the error ARE there.

I found a bunch of posts with the error for files that aren't there, but couldn't find anything for the same error when the files definitely are present.

I wondered if anyone has a solution. I did add an exclusion to the backup, but they don't apply to VSS.

Thanks.