Hello there.
Can someone help me guide this issue? I have just installed a fresh Nextcloud server in Docker with Letsencrypt. I was using Techno Dad Lifes videos. After getting the instance on the internet, Nextcloud is telling me there is an issue with the reverse proxy configuration.
I have searched the web for similar issues, but can’t seem to find something that solves mine.
The message below is from Nextcloud admin settings --> Overview. This message is also translated with Google translate from my language to English. I hope you will understand:
The reverse intermediate server head is not set up correctly, or you are connecting to Nextcloud from a trusted intermediate server. If not, this is a security issue and may allow an attacker to falsify their IP address as it is visible to Nextcloud. Further information can be found in the documentation.
Crossing my fingers you help me out of the issue! 
(Nextcloud 19.0.0)
Best regards
Sputnik1