Problems external storage with webDav on https:

Hello, i just installed Nextcloud VM from in my QNAP NAS and it works.
Now i want to sync a folder from the NAS to Nextcloud as an external storage. It works only with http: but not with https:. Ports are forwarded properly. Is that a problem of certificates? Any help?
Thank you!

I would recommend to use the search function of the forum to find an answer. In most case this kind of problems has already been addressed in the past: