404 on post pages for font files

This is a new WP installation, still in development, not yet production. I modified 404.php to email when the page was invoked. Then I started getting a lot of messages that specific requests are invalid:

/wp-admin/400&latin?wp-mce-4208-20151113
/wp-admin/latin-ext?wp-mce-4208-20151113

The only pages triggering this are:

/wp-admin/post.php?post=0000&action=edit
/wp-admin/post-new.php?post_type=page

So the 404 is just on post creation/editing, not from the front-end. That makes sense since it looks from above like this may be related to MCE.

I don’t have any plugins for fonts. The theme (Layout Builder, child of SG Windows) does support Google fonts and offers huge selection for the front-end, but that shouldn’t apply to admin (right?).

At this time I know nothing about how to work with fonts in WP but I’d like to learn. I’m developing in USA/English and have no need for Extended Latin right now. However, I’m building for an international audience, and it’s multi-site, so site owners will have access to some admin features and I do need to understand this font issue.

Thanks!

EDIT: I’m starting to think this might be related to some other plugin that adds functionality into post maintenance, like Shortcodes Ultimate. I’ll look into that.