WOPI URL results in http even though https is configured

[jonaskohl]

I tried setting up the Office Online extension. I added the Collabora service and put it behind my reverse proxy (under gwoffice.infra.<DOMAIN>.de) and added this URL to the settings:

But when I try to edit an Office document, the resulting page (https://groupware.<DOMAIN>.de/wopi/...) will have that URL with http instead of https.

...
<form id="office_form" name="office_form" target="office_frame"
      action="http://gwoffice.infra.<DOMAIN>.de/browser/9f059c5093/cool.html..." enctype="multipart/form-data" method="post">)
...

In my nginx config I correctly set the protocol to https via proxy_set_header X-Forwarded-Proto https;.