The error "The process cannot access the file because another process has locked a portion of the file. ((E:\folderpath\file.pst)" is being reported when backing up with a DS-Client installed on a different computer than the backup target computer.
Caused by the fact that the "Remote Registry" service was disabled and stopped on the backup target computer (this is a default state for this service when initially deploying Windows 8/8.1 operating systems).
On the backup target computer go to Start > Administrative tools > Services, set the "Remote Registry" service to "automatic" and start it, then try the backup again from the remote DS-Client computer. The file system needs to have the "Backup through volume shadow copies" option selected in backup sets properties, advanced options.