In reading the issue and examining the code the problem is that WSGIDav examines the requested scheme (http or https) vs the actual scheme. In otherwords, when you contact the server you use https://server.org. This request is SSL terminated by the NGINX reverse proxy which forwards the request as http://127.0.0.1:8080. The headers passed along all indicate a scheme of HTTPS, but the actual communication is on HTTP (non-SSL). The WSSGIDav code sees the incongruity and rejects the request with the 502 error.
This is correct behavior according to the standard. The issue is the code should be checking the X-Forwarded-Proto and then act accordingly, but currently does not. So adjusting the X-Forwarded-Proto will have no affect at the moment.
The only other options are to rewrite the headers so it looks like the original request was HTTP or to enable HTTPS on the WebDav backend - neither of which is trivial. Or you just do not use a reverse proxy.
As I have time I’ll research options and see if a solution presents itself.