HTTPS for Paperless-ngx using Nginx Proxy Manager

[RobertJC55]

Issue Summary:
I'm trying to set up HTTPS for Paperless-ngx using Nginx Proxy Manager with a custom/self-signed SSL certificate on my local network.
Setup:

Paperless-ngx running in Docker (container name: paperless-webserver, internal port 8000)
Nginx Proxy Manager in Docker
Both containers connected to the same Docker network (paperless)
Custom SSL certificate (self-signed, signed by my own CA)
Domain: paperless.home (local DNS)

Problem:

HTTP works perfectly: http://paperless.home:8090 loads Paperless fine
HTTPS gives 403 Forbidden: https://paperless.home fails
Direct IP access works: http://192.168.68.150:8500 works fine
NPM can reach Paperless container (verified with curl from inside NPM container)
The 403 happens ONLY when SSL certificate is enabled in NPM
Proxy host logs (proxy-host-26_access.log and proxy-host-26_error.log) are completely empty - no requests logged
Fallback logs only show nginx warnings, no 403 errors

What I've tried:

Disabled "Block Common Exploits"
Disabled "Force SSL"
Set Access List to "Publicly Accessible"
Recreated the proxy host from scratch
Cleared browser DNS and cache
Tested from multiple devices/browsers
Verified Docker networking (curl works from NPM container)
Tried uploading certificate and key separately
Tried combining cert and key into single PEM file

Configuration:
Forward to: paperless-webserver:8000 (http)
SSL Certificate: Custom certificate (self-signed)
The moment I select ANY SSL certificate in NPM, it returns 403. Without SSL selected, HTTP works fine.
Has anyone seen this before? Is there a known issue with custom certificates in NPM?

Thanks for your ideas and assistance!

Robert