KB2027: Job reports warning "Failed to truncate ...
https://www.veeam.com/kb202728.04.2015 · To investigate why SQL log truncation has failed, the Veeam logs will need to be reviewed to determine which part of the truncation procedure failed. For Veeam Backup & Replication, the log file containing information about the truncation procedure will be found on the SQL server, is named: C:\ProgramData\Veeam\Backup\VeeamGuestHelper_ddmmyyyy.log
SQL Log Truncation and Log Backup ... - Veeam Software
https://www.veeam.com/kb244723.02.2018 · Challenge. After installing Veeam 9.5 Update 3, SQL logs are not truncated during backup, but the truncation task is still marked as a success in the job statistics. SQL Log Backup sessions complete with 0 KB transferred. You can confirm if any SQL VMs are affected by this change by checking the Guest Helper log located on the Guest VM.