I opened the developer console and found an error that’s relevant:
Mixed Content: The page at 'https://cloud.domain.com/apps/files/?dir=/Documents&fileid=122' was loaded over HTTPS, but requested an insecure form action 'http://cloud.domain.com/custom_apps/richdocumentscode/proxy.php?req=/loleaflet/ed4f732/loleaflet.html?WOPISrc=https%3A%2F%2Fcloud.domain.com%2Findex.php%2Fapps%2Frichdocuments%2Fwopi%2Ffiles%2F126_ocbttb0r5cs1&title=Welcome%20to%20Nextcloud%20Hub.docx&lang=en&closebutton=1&revisionhistory=1'. This request has been blocked; the content must be served over HTTPS.
(I replaced my domain). Any ideas? I’m using the built-in CODE server and I’m updated to 19.0.2. This seems like a bug.