It seems you are doing backup full and differential on the ReportServer database. That database is the reporting database and the default recovery model for it is Full, which means
you need to backup the transaction log of that database.
Full recovery model rely on transaction logs to provide full recoverability, preventing data loss and allowing you to recover to an earlier point in time.
With full recovery model, transaction log backups are needed not only to decrease your risk of losing data but to truncate the inactive portion of the transaction log. SQL Server reuses this truncated inactive space of the transaction log instead of allowing
the transaction log to continue grow and use more storage resources. If you don’t backup the transaction log, you may run out disk space.
In my opinion, BackupExec is a great choice for doing SQL Server backups, and can replace SQL Server internal backup tool.