Problem: When backing up MS Outlook ".pst" can't be backed up while MS Outlook is running.
Solution:
This problem is connected with the lack of admin rights. MS Outlook doesn't allow to backup .pst files while running if you don't have sufficient rights.
Make sure that you are running Handy Backup with administrative rights (right button click on the icon -> Run as administrator) or your user account has admin rights (included in the admin group).