Webdav External storage config - Issue with HTTPS

Good day,
I have installed Nextcloud on my Synology DS218Play and I am currently setting the External Storage through WebDav for synchro of my NAS.
The config works when I put the HTTP port however, it is impossible to make it work with https port. It becomes impossible as well when I set it up through the HTTP port and tick the HTTPS securisation box.
What kind of problem can it be? Is it something related to the certificate?
For info, Nextcloud is already forwarding all HTTP to HTTPS
Thanks a lot for the help.

How did you mount the external drive? - Which permissions are set?
The external drive needs the same permissions as the Nextcloud data-folder.

Thanks for the reply.
I installed it through web station, MariaDb anf apache.
Which permissions have you in mind? You mean port forwarding, firewall etc?

Did you mount the WebDAV drive using https (SSL)?
Are the file and folder permissions the same as the permissions in Nextcloud datafolder (on most systems www-data.www-data)?