To find out which program blocks your pst files, download the program named "Unlocker". After installation or better directly after getting one of the following error messages:
"Store does not exist"
"Folder does not exist"
"The folder X is not supported"
open the folder where your pst files are located (normally "C:\documents and settings\User account\local settings\application data\Microsoft\Outlook"). Right-click your pst file indicated in the error message and click "unlocker". The program will tell you which programs/processes access the pst file at the moment. If there is another program shown as Outlook itself and SYNCING.NET, please deactivate this program (if possible). If there is no other program, please check and repair your pst file with OLFix.