Instead of the normal page flow that takes you to the download backup file page, you are presented with a 404 error. This happens whenever you try to create a database backup of a SQL Server and you have a space in the name of the backup.
To make sure that this KB is related to you follow the steps:
Create a SQL Server Database and populate it with data
Go to the database backup repo ( Subscription->Websites & Domains -> Bacbup Manager -> More Actions -> Database Backup Repository ).
Back up an MSSQL database (Make sure there is a space in the backup name)
Click on the OK button to submit the backup.
As a result 404 error page should be shown.
It is software issue PPPM-2724
NOTE: It's non critical/blocking, as the backup does get created, and you can restore from the backup.
For Plesk 12.5 this issue does not apply since database export/import functionality has been rework.