Nginx Error when creating SSL Certificate

I have Nextcloud running as a Docker on an Unraid Server as well as Nginx-Proxy-Manager on Port 80 and 443.
To Access my Cloud from the Internet, I exposed my device via AVM’s ‘MyFritz’-Service, so I got an URL such as ‘’ pointing at my Server. Thats working fine so far.

I also have a URL with a CNAME Record pointing to my Fritzbox, which is also working fine (its showing the ‘Congratulations-Page’ from the Proxy Manager).

So I went and created a Proxy Host with my domain pointing to my nextclouds IP and Port.
Without an SSL-Certificate I get “ERR_EMPTY_Response” in Chrome. This happens when I try to go to as well as and https://… If i type “” I am redirected correctly to my nextcloud Login-Page, i cannot login tho.

If I try to create an SSL-Certificate with Nginx-Proxy-Manager it says “Internal Error” (which is not a surprise since the URL doesnt work properly).

I would be very thankful if someone has an Idea on how to solve this Problem because i havent anymore.
Is it because Nextcloud only accepts https? Do i have to change that while creating a Certificate? Are there any other things I have to change in my config apart from Trusted URLs?

I did this whole setup with another version of nextcloud about a year ago and it worked fine…