@andrewweb the icons are all from fontawesome. You don't want to edit any css files. And if you edited admin.css, sure the icons might show up there, but they wouldn't show up on your forum. You can try adding custom css at ACP > Customize > CSS. That might work.

@andrewweb So... I uninstalled sendmail from the server, even though I wasn't using it, and then changed something in UFW and now the board works. I'll see if the issue with registration and password persists, and I'll open a issue on GitHub. Thank you both for your kind answers.