To end that long thread, with the help of @JackGruber, here is the solution to make Joplin work on my local network.
My network diagram is like the following:
FACTS :
- Reverse Proxy and Nexcloud server are on the same box (2 docker containers).
- Certificate is installed on Reverse Proxy (nginx).
PROBLEM:
When using webdav URL provided by Nextcloud, I access directly to Nextcloud server (port 443) and therefore bypassing Let's Encrypt certificate. Thus the issue "Networkd request failed".
SOLUTION:
I modified the Webdav URL from
https://nextcloud.my.domain/remote.php/dav/files/username/joplin
to
https://nextcloud.my.domain:444/remote.php/dav/files/username/joplin
Specifying port 444 makes the Joplin sync work !
-=-
I hope this will be useful for someone else. Big thanks to JackGruber !