When after the initial scan of files to be backed up, a file was removed or deleted that was part of that initial scan. This is typically related to selecting temporary folders such as Temporary Internet Files which can automatically removes files.
Review, if not a fully re-select, the files for backup. Ensure only data you frequently use is being selected and not any temporary folders. Attempt to backup your data again. if the issue persists you may need to request a rebaseline, which will clear out the past backup history and allow you to start with a fresh backup. You will need to contact support to make this particular request.
If you continue to encounter an error after completing the above steps, please contact 1-800-656-6115