Hi Dean,
Are you using Object Consolidation and want to archive Virtual Full Backups?
From the given path it looks like you are performing a backup into a DP File Library with option DFMF enabled.
DFMF creates a quite complex file strucure to support the Virtual Full backup feature. You should not delete any file manually in such a configuration as these files depend on each other and you might risk to loose all data in that file library.
In case you activate that option DFMF for the File Library, but do not use Object Conolidation or you wanto to run only Syntehic Full backups I recommend to quickly deactivate this DFMF option
With the given information I cannot say how to fix the current issue.
As a quick shot you could check if you modified the associated File Library Pool from "non appendabe" to "appendable".
Default is non appendable!
Best Regards,
Joerg