The dloclientu.log file at "Enable Verbose Logging" will have this line in it, which indicates the DLO Agent failed to read the required registry keys as noted below:
dloclientu.exe 4216: W> mapitools.cpp( 1327 ) MapiRegistryHack() - rename failed... E0140005
To turn up DLO Agent logging verbosity:
1. Launch the DLO Agent GUI
2. Select Tools > Support > Enable Verbose Logging
3. Restart the DLO Agent (don't just minimize it and maximize it)
This issue can occur because the Microsoft API which the DLO Agent uses to backup PST files incrementally expects the presence of, and access to, certain registry keys.
Within this registry path:
HKLM\SOFTWARE\Clients\Mail
These two keys must exist with these values:
(Default) = Microsoft Outlook
PreFirstRun = Either there is no default mail client or the current mail client cannot fulfill the messaging request. Please run Microsoft Outlook and set it as the default mail client.*Microsoft Office Outlook

To resolve the PST access errors in MS Outlook, the DLO Agent must be able to backup the PST files using the VSS snapshot methodology as opposed to using the alternative MAPI methodology.
1. Validate the registry on the problematic desktop machine contains (Default) and PreFirstRun and that the values are set correctly as stated above.
2. Validate the user has the needed permissions to modify both of these keys. i.e. change 'Microsoft Outlook' to 'Microsoft Outlook TEST' and change 'PreFirstRun' to 'PreFirstRun_TEST'.
NOTE: Please change the keys back to their pre-altered values, once the permissions test has been run.
NOTE: Two other root-causes known to produce similar PST access problems within MS Outlook are:
1. Delta File Transfer not enabled in the PST Backup Selection for the Profile in question.
2. The DLO Agent bittenss does not match the bittness of MS Outlook. For instance, 32-bit DLO Agent backing up PST files configured in 64-bit MS Outlook.