Hi Erick,
Thank you for your reply. Here are my answers to your questions:
1. I am using Safari, but tried with Google Chrome and the problem persisted.
2. I am not aware of any of my colleagues having the same problem, and I have no colleagues knowing the same contact so can't try searching for it on another account.
3. If I search this person's e-mail, I can see the person as a contact (name and e-mail address), and i can add this person.
4. All e-mails of this contact show correctly, but there is one item at the top of the list with no information other than the date and time, which is approx. 12 hours after the last e-mail was sent to this address.
I hope this helps you. Thank you for your help.
Best wishes.
BBth