SQL Server Backup failing for just 1 DB ("The system cannot find the path specified.")
Ok i finally figured out the issue. It was completely random that it happened on the ReportServerTempDB, and i think that's what threw me off.
My daily maintenance runs at 3AM and includes the backups as above. I also have 30min log backups happening 24/7, also with the specified 'Y' drive and deleting it.

The Log backup was running during the normal backup, and deleting the 'Y' drive from the full backup, therefore killing the full backup as the location now no longer exists.
I have added some logic in, so that the Log backups do not remove the drive between 3AM and 6AM and that's solved the issue.
Thank you all for the assistance though.