Unable to create Task in Outlook

When in Outlook, one client gets the message “Could not complete the operation. One or more parameter values are not valid.”

She is on Win98SE and was fine until 3 or 4 days ago, which unfortunately appears to have no reasoning behind it. Nothing about her machine has changed, and I get it on my NT machine when I connect to Outlook using her mailbox. I am assuming it is the original mailbox on the server because of this, but I don’t fancy re-doing her mailbox again. Has anyone seen this before and is there a setting or file that may be causing this??

Thanks in advance.