by Dani Palou.
Hi Harry,
your problem is not related with the certificate, it's a bug in Moodle 3.3.
One of the problems that can cause this error was fixed in Moodle 3.3.2, so please try updating your site to that version. If you're already on that version or it doesn't fix the problem, please try applying the patch in this issue to your site:
https://tracker.moodle.org/browse/MDL-60132
This issue will be fixed in Moodle 3.3.3.
Kind regards,
Dani