[Nginx Proxy Manager & Cloudflare]

I am not too familiar with Fritz box but I think you have the ports mixed up. You need the external port (WAN side) to be 443 and the internal port (the one pointing to NPM, LAN side) to be 18443

[Nginx Proxy Manager & Cloudflare]

So there should be no SSL until you hit your NGINX Proxy Manager instance. If you can't get HTTPS to work make sure your firewall is appropriately port forwarding to your NGINX Proxy Manager SSL port.

The forwards should look something like this if you are using the Proxy Manager App in Unraid:

 

Firewall  -> Internal Port

80 -> <ip of unraid host>:1880

443 -> <ip of unraid host>:18443

[Nginx Proxy Manager & Cloudflare]

It sounds like you are terminating the SSL endpoint with cloudflare using the WITH PROXY. This will use the IP address of one of the cloudflare servers. You are then trying to SSL back to your Unraid Proxy Manager. So cloudflare may be giving that error because cloudflare is not trusting your certificate by let's encrypt because it doesn't have it stored (which I think cloudflare handles on it's own).

 

So it sounds like your setup looks like this:

 

subdomain.example.com:443 -(ssl)> cloudflare -(ssl)> proxymanager:443 -> subdomain.internal:8080

 

I am currently trying to work on a similar solution but haven't gotten as far yet. If you want to try and test this out switch the cloudflare DNS setting to DNS ONLY instead of WITH PROXY and see if that works out.

 

That setup would like like this:

subdomain.example.com:443 -(ssl)> proxymanager:443 -> subdomain:internal:8080

 

Some one with more experience may be able to set me straight as well though.