Backup suddenly failed - no error log?

I realized that for the past three months my backup has not been running!
When attempting to run my main backup manually I get:

“The process cannot access the file because it is being used by another process.”

I have not made any changes since this backup last worked 3 months ago.

I am running

When I click on “show” on the error I only get a log from December 2018, but at this time I believe the backup was working.

Help would be greatly appreciated, it’s unnerving having a backup system suddenly fail.