Hi,
Thank you for posting your query,
If we have understood your query correct then it appears that there is an issue with Tape 1. As you tried changing the Tape from Media set 1 to Media set 3 still the backup job works fine. It clears about the Media set settings.
We would suggest you to use new media/tape in remaining slot and try to perform backup.
If the Backup job still goes into queued state then drag and drop that particular media manually in Scratch media set and then target the Backup job to the same.
Verify the result.
Hope this helps.
We hope this information helps, if you have any other questions please respond to the post and we will be more than happy to assist you further.