This one looks like the most recent backup developed a problem. You can check that against the “Restore from” dropdown in the GUI for that job, to see if 0: and date match. If so, then backup the database (if you haven’t already), finding it by “Local database path” under the job Backup option. This copy is just-in-case.
Next step is The DELETE command which is probably most easily done using the job Commandline option. Change the command dropdown at screen top to delete, delete all the source paths from Commandline arguments box, Near bottom, Add advanced option of version, find it, set it to 0, then at bottom Run "delete" command now. This should delete your most recent backup. I’m assuming the database is still seemingly good except for the fileset error. If you know of other issues, then the plan should be rethought.
Being extra careful would have you first do a run using the –dry-run and review the output for any worries. Ordinarily, deletes occur all the time unless you’ve got Backup retention at Keep all backup. I guess might as well be cautious and at least see that delete wants to take out only a dlist of the right time along with whatever dblock and dindex files it might have added. That should take only a quick look at its output.
Using the Command line tools from within the Graphical User Interface gives further info on Commandline.