Never seen it. Several things to try, not in any particular order...

Recreate the Outlook profile (don't use the copy option, manually recreate it).

Recreate PST, move items from old to new.

Check for office updates, install if applicable

Depending on the OS version, perhaps recreating the local windows user profile...don't forget to save a copy of the profile, I usually just rename the old one, then have the user login to make the new one, do what you need to test out this issue, if the problem persists, you can always go back to the old profile.

Check permissions on the user profile, the default temp directory for outlook…

Try deleting all of the temp files, and temp Internet files…

Rename Normal.dot

Then, shoot the clerk.