Old, broken *.github.com Github Pages links
Some pages on kwark.info link to links hosted on GitHub Pages using a URL on *.github.com.
Unfortunately, GitHub recently removed the automatic http[s]://.github.com/ -> https://.github.io/ redirection that they'd had in place for years for some reason, so to fix those links you'll need to rewrite them.
For example, https://gl.kwarc.info/kwarc/kwarc.info/www/-/blob/master/systems/omdoc.md has links to https://uniformal.github.com
which need to be updated to point to https://uniformal.github.io
; there may or may not be others on other pages.