Readers not able to comment

Readers who previously were able to comment on my blog now can’t. They get an error saying they have not filled in the name/ email addy sections.

I have the box ticked to requite that, but I have always had that box ticked.

The only thing that is new is that my version of WordPress has updated. I wondered if my theme was now incompatible so I have installed twenty fifteen, but still no joy.

But I can comment, logged in to the WP account for the blog, a different WP account, and not logged into any account.

The only plugin which seems to have anything to do with commenting is Jetpack – I have the comment section of that enabled.

Help?