First make sure the user is not over their quota and time zone is correct.
Go to Run and type:
ensured that the checkbox for “adjust for daylight savings time” is checked
Delete the frmcache.dat file
Rename the ost-file to .old when Outlook is closed. Start Outlook again and have it rebuild its cache.
recreate a new PST for the outlook profile by FILE->NEW->OUTLOOK DATA FILE. create new pst . copy all the folders from old PST to the new one. Close the old PST from outlook. restart outlook . (before you copy old folders to new , do a repair of the Old PST using SCANPST.EXE. do a search to get this exe inbuilt in office). The cause for the error is probably due to comaptibility issue of the PST due to non unicode content .
To test if this solution might work Close all PST from user outlook profile by right clicking and close and then try test calendar or appointment request.
If the above solution does not work try removing microsoft live meeting /conference addin.
Check if the user is using blackberry or similar services.
Check if the user has set delegates which has set automatic calendar acceptance.
Check if user has configured outlook on another workstation.
check if User has set internet calendars (other than exchange server settings)