It seems another program blocks your pst file. Do you use additional software for Outlook doing backups or accessing the pst file permanently (e.g. ActiveSync)?
If this error appears repetitively, you should:
1. Remove the SYNCING.NET "Outlook Group" and exit SYNCING.NET.
2. Check and repair your Outlook profile(s) and pst file(s). Use OLfix for this.
3. Launch SYNCING.NET and create a SYNCING.NET "Outlook Group".
It is also possible that your hard drive has some errors on it. Use "chkdsk" to check and repair.