One of the my blogs stopped allowing comments. If you experience this is might be due to an issue with https. The blog in question was serving https pages and everything had been working previously. I am not sure when the issue first appeared, but I noticed it after updating to WordPress v 5.1.
When I looked at the page source I could see the comment form was trying to submit to http (even though it was being submitted from a https page). I received a message that the page was going to be submitted in an insecure manner and did I want to submit anyway. I said yes and a new page was loaded (though it was blank). But no comment was submitted to the wordpress database.
Using another browser it gave a 405 error.
This page isn’t working
If the problem continues, contact the site owner.
HTTP ERROR 405
I couldn’t figure out why it was submitting to http. I remembered there is a general settings page (/wp-admin/options-general.php) and I went there and noticed the
WordPress Address (URL)
Site Address (URL)
fields were set to http. I updated them to https and then everything worked fine.
Related: Compare WordPress Files on Server to Proper WordPress Version – Fix for When a WordPress Blog Stops Displaying Images – WordPress: Multiple Blog Network on One Server – Overcoming Conflicts