BWC Agreements - Approval Options: 'No items found' when searching for Approver

One of our end users has an issue in Buyer Work Center.

When she has updated an agreement and clicks on the button 'Approval Options', to search for a specific Approver, the search gives the result: No items found (search on 'Last Name').

Her colleague performs the same actions, and is indeed able to find the specific Approver and can continue with clicking on the button 'Submit Document'.

We have asked our end user to log off and/or clear the cache, delete cookies and start a new session.

But that did not help unfortunately.

This problem does not occur with all agreement updates, but once in a while.