Hey,
qutebrowser upstream here - I recently looked into this a bit, and partially
found out what's going on.
Turns out when supposed to display an error page, "jstProcess is not defined"
gets logged instead.
When I look at the source of the error page on my Archlinux machine, that
_javascript_ function is properly defined, like it is upstream here:
https://cs.chromium.org/chromium/src/third_party/jstemplate/jstemplate_compiled.js?q=jstemplate_com&sq=package:chromium&g=0
However, on Debian, the related code is simply missing from the error page.
I'm guessing this is somehow related to how Debian regenerates that file from
sources:
https://salsa.debian.org/qt-kde-team/qt/qtwebengine/blob/debian/5.12.5+dfsg-3/debian/rules#L135-141
Maybe there's something going wrong there, and the generated file is actually
empty or something?