The weirdness is in fact the error comes randomly O_o
Steps to reproduce: continue pressing a "Page Reload Button" (Ctrl + R?), one per 10-20 attempts you'll catch the error. No matter what page you are on, even ACP.
Just a quick update, @psychobunny ended up taking the lessons he learned writing the sidebar in Persona, and wrote another one for our new marketing site, and just today, spent a bit of back and forth with @Schamper to debug it.
He's planning to backport it to Persona, so... soon! 😄