Discovered by rezeau while testing MDLQA-18818.

The problem happens when 'Display H5P' filter is enabled and a page resource has a valid h5p wordpress url such as https://generic.wordpress.soton.ac.uk/altc/wp-admin/admin-ajax.php?action=h5p_embed&id=1, when viewing the content the wordpress url shows the following error "Firefox can't open this page":

This can also be reproduced in Chrome, although in that case, the error is uglier:

For QA 4.3 they were passing (there is a screenshot in MDLQA-18327), and in https://sandbox.moodledemo.net/mod/page/view.php?id=7&forceview=1 (which is using Moodle 4.3), the error can be reproduced too.