If you create the link using the Article button below the editor, do you get the same 404 result?

I don't have an article button below the editor

Please send me a login - https://www.joomlacontenteditor.net/contact/site-login

You have to create a menu item for each article you link to, otherwise the menu association (itemid) defaults to the Main Menu, and may result in a 404.

I have created the menu item for the article in question on your site, and updated the Next link with the new url.

That's not a good option as I'll have 6 submenus on my site.

These articles have been on my website for over 5 years. It's only when I went into it to change some text they stopped working. I've had quite a few gremlins like this since I upgraded to JCE Pro. Joomla should or did use to allow direct link to article content.

I'll probably just create a page break and condense the 6 articles I have into 1.

I've got quite a big site with lots of articles. I'm concerned it'll be broken elsewhere that I don't know about.

I've disabled the menu and reverted the link.

JCE using the Joomla API to create the links, so they are assumed to be correct. The Joomla Menu / SEF URL system is a complicated mystery, so JCE relies on the API to produce accurate links.

I've added the Article button at the bottom of the editor, which is a Joomla Core button for adding links to articles. If you use this to create the Next link, it creates an almost idential link (same id and catid but omits the alias values), which gives exactly the same 404 result when clicked.

Confused this doesn't work either