I upgraded one of the user's computers from Outlook 2002 to Outlook 2013, retaining the same Outlook.pst file

We attempted to clean up her Sent Items folder but she now has items in her Sent Items folder that cannot be removed. She gets the message "The Operation Failed. The messaging interfaces have returned an unknown error. If problem persists, restart Outlook" Restarting Outllok has no effect.

Strangely, items that she sends now do not go into her Sent Items folder, but instead are placed in her Outbox - yes they are sent also but remain (?) in the Outbox.

We have run Scanpst to try to correct any error in the Outlook.pst file.

I'd appreciate any help,