Logs are not truncated on Microsoft SQL server after a successful backup.
The following error can be found in the Windows Application log or in VeeamGuestHelper.log file (in earlier versions this log file is called VeeamVSSSupport.log) located in C:\ProgramData\Veeam\Backup on the guest:
The server principal "NT AUTHORITY\SYSTEM" is not able to access the database "Database name" under the current security context.'
The server principal "domain\username" is not able to access the database "Database name" under the current security context.'
Depending on Veeam B&R version there may be different causes:
In Veeam B&R prior to v8:
By default, LOCAL SYSTEM account does not have log truncation rights on SQL 2012.
In Veeam B&R v8:
Starting with version 8, Veeam uses the account specified on “Guest Processing” tab in the job settings for log truncation. This user account may not have required user permissions for performing database backup or log truncation.
Depending on Veeam B&R you need to apply the following settings either for NT AUTHORITY\SYSTEM (pre-v8) or for the user specified in the backup job (v8):