You won’t believe it. To fix it, all I did was empty the content of the “Proxy Extensions” field, save it, and right after that, the page started working.
The remaining extensions are jpg, jpeg, gif, png, ico, svg, css, zip, tgz, gz, rar, bz2, exe, pdf, doc, xls, ppt, txt, odt, ods, odp, odf, tar, bmp, rtf, js, mp3, avi, mpeg, flv, html, htm.