The error in the admin interface when trying to install the certificate let to a break down of the SSL parts of the system. I noticed that only very late because it became apparent only after a Zimbra restart. I managed to get the system back running by installing a new self-signed certificate. In the course of doing so I noticed that
SSL Certificate Problems - Zimbra :: Wiki is horribly outdated and the certificate installation program is not documented in the official documentation. So fixing the system was more or less guess work. Not very reassuring!