Org Mode hangs when opening Agenda

Question

I'm having an issue with Org Mode whenever I open the Agenda.

Emacs gets stuck calling Contacting host: fniessen.github.io:443 (seems like an Org theme repository).

This same hanging also happens sometimes when I open timestamps on Org Mode.

What can I do to debug / prevent this from happening?

Update

After running pkill -SIGUSR2 emacs I got the following output (debugger):

Debugger entered--entering a function:
* symbol-overlay-refresh(1 5 0)
  url-retrieve-synchronously("https://fniessen.github.io/org-html-themes/setup/t...")
  org-file-contents("https://fniessen.github.io/org-html-themes/setup/t..." :noerror)
  org--collect-keywords-1(("SETUPFILE" "FILETAGS" "TAGS") ("ARCHIVE" "CATEGORY" "COLUMNS" "PRIORITIES") nil ("/home/user/dev/emacs/reports...") nil)
  org-collect-keywords(("FILETAGS" "TAGS") ("ARCHIVE" "CATEGORY" "COLUMNS" "PRIORITIES"))
  org-set-regexps-and-options(tags-only)
  org-agenda-prepare-buffers(("~/dev/emacs/reports/..." "~/dev/emacs/contacts.org" "~/dev/emacs/emacs.org" "~/dev/emacs/todos.org" "~/dev/emacs/notes.org" ...))
  org-agenda-prepare("Day/Week")
  org-agenda-list(nil)
  funcall-interactively(org-agenda-list nil)
  call-interactively(org-agenda-list)
  org-agenda(nil)
  funcall-interactively(org-agenda nil)
  call-interactively(org-agenda nil nil)
  command-execute(org-agenda)

Answer 1

Turns out one of the Org files in my agenda has a #+SETUPFILE path that doesn't exist:

#+SETUPFILE: https://fniessen.github.io/org-html-themes/setup/theme-readtheorg.setup

had to be changed to https://fniessen.github.io/org-html-themes/org/theme-readtheorg.setup

The debugging information was extremely useful for solving this case.