This kind of trouble started during the weekend, outside the office hours of my web space provider. Once I called them (during their office hours), they provided me with the respective lines from the apache log, and I was able to get my problem solved rather quickly.
The “jetpack” directory was rather incomplete – maybe that resulted from silently getting my disk quota exceeded during a half-automatic self-upgrade of the WordPress plugins.
During sorting this problem:
- I got aware of WP_DEBUG within wp-config.php, and I set it to true temporarily; that way I am able to see PHP warnings myself, not just within the apache log, that I have no access to
- I decided to get my web space disk quota upgraded; I am fed up with this silly, silly trap; it is so easy and cheap to get avoided.