What happens if you add this to the end of the image handler URL

?page=about

so it reads

image_handler.php?page=about

That should emulate the link that is being used and might shed some light on the issue.
Absolutely nothing happens. I had already tried it with all four links.

However, typing this in the URL does respond and clears image cache: https://www.example.com/STORE_NAME/M...ih_clear_cache

This typed directly: https://www.example.com/STORE_NAME/M...ion=ih_install responds with the message "Image Handler successfully installed" but nothing else changes in terms of functionality.

Any clues from that?