Luckily there is a Veeam KB article on for this error which can be found here.
Unfortunately, the solution provided ‘ Start the Backup Job (right click the job > Start).’ didn’t fix the problem. Well to be precise, it worked for about 24 hours and then the error returned.
Further reading the KB article explains that “Native SQL Backups cannot be taken in conjunction with Veeam SQL Log backups unless they are COPY ONLY. Multiple applications interacting with the Transaction logs will break the consistency and prevent proper restores.”
Which also appeared to be the root cause of the problem experienced by the another Veeam user here.
So firing up SQL Management Studio and connecting to the SQL server in question, I found under ‘Maintenance Plans’ two tasks, one called ‘Clean_Up’ and the other ‘DB_backups’.
Editing ‘DB_backups’ show a backup database task configured as a ‘Transaction Log’ type which was set to run once a day and ‘Copy-Only Backup’ was not enabled. After enabling the ‘copy-only backup’ setting and scheduling a quick backup in Veeam (this creates a Full SQL backup so the transaction log backup had something to reference) the error has never reappeared.