Over the past couple of days I've noticed a problem when posting a comment on the blog.
The comment is submitted but you are not directed back to the post. No follow-up actions occur either such as sending a webmention where appropriate or mailing me to advise that a comment has been submitted.
After checking, I saw the site is generating a 405 error in wp-comments-post.php which checks that the request method is POST and fails if the method doesn't match.
I'm not sure when it started but I think it might have coincided with the update to WordPress 4.9.7.
I tried disabling a number of plugins and custom functions to no avail. Searches revealed that this is a relatively common issue with some saying it relates to a problem sending emails from WordPress. I turned off the email notification options in Discussion Settings and comments started working.
Email is definitely the issue. I've tried a couple of SMTP plugins instead of relying on defaults but get the same error so I don't know what's actually causing it.
I've left the email notifications off for now so that things work until I can find the cause. There may, consequently, be a delay in me responding or reacting to comments.