Latest WP update breaks Classic Widgets and more

This is basically just guesswork on my part, but the latest update to WordPress seems to have broken Classic Widgets but also Classic Editor and is causing issues in Elementor (probably because the Text Editor widget still uses the classic editor).

So what happened was I made some changes to a HTML widget in the footer of the link attached, but after I hit save the loading icon just kept rotating for a long time and nothing got saved, tried it 3 times.

Then I disabled the Classic Widgets plugin and used the new Guttenberg one and it saved the same changes instantly.

I only mention the following because it seems related, otherwise it would be off topic.

A similar issue cropped up in pages/posts edited with the Classic Editor plugin, however this still saves changes, unless you have some styling (like different font, font-size, underline, etc… anything using the span style code) or if you center text, other code like images, links don’t cause issues.

If any of the above mentioned code exists in your content then when you save/publish you are sent to a Not Found page instantly and all your work is lost.

A similar thing happens with the Text Editor widget in Elementor, there having that code in your content results in a Server Error (404 Error) when trying to save changes.

I haven’t tried disabling all other plugins yet so I didn’t report the issue to Elementor, Classic Editor or WordPress yet, but the non-widget issues occurred on at least 4 websites already.

The page I need help with: [log in to see the link]