The reason is that URLs like this:
https://www.xwiki.org/xwiki/bin/export/Main/Support?format=html-print&xpage=get&outputSyntax=plain&async=true&sheet=XWiki.PDFExport.Sheet&jobId=export%2Fpdf%2F1711446569226-223&
are blocked by the front-end (403 Forbidden), even if I'm logged in.