Any leads to ERR_CONNECTION_RESET

I tried putting a script for an embeddable form in a page on the site I’m working on, and it gives me a a browser/server error ERR_CONNECTION_RESET. I assumed at first this was a result of me submitting the script where it is not appropriate. I tried instead to add it to a plugin I made for the site’s custom shortcodes, and got the same error.

With a bit of experimenting I found that basically any <script> tag will cause this when submitted in any form I can think of – article, page, plugin editor, etc – regardless of what it contains and even if it is totally empty.

What’s causing this? I vaguely recall script embedding issues before but nothing this extreme.