External Storage (WebDAV) no https

Nextcloud version (eg, 18.0.2): 19.0.3
Operating system and version (eg, Ubuntu 20.04): Synology DSM 6.2
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4
PHP version (eg, 7.1): 7.3

My Nextcloud Instance is available under mycloud.mydomain.com as a vHost.
I have added some folders on the same server as external storage via WebDAV.
If I use the same URL, Nextcloud uses, http is fine (mycloud.mydomain.com:5005) but if I use https (mycloud.mydomain.com:5006) and activate the https option, the external storage turns red.
If I change the URL to some other subdomain that point to the server (myserver.mydomain.com) https works fine.

Is https not working because it is the same host?
Is there an issue?
Is it save to use http as the folder is on the same server?
If I use https and a different URL are requests between NC and the shared storage handled directly on the server or are the goinig through the router?

If I use the same URL and https, the log just says the mount ID X is not available but no additional error message.