Issue running pgAdmin behind a reserve proxy

Hi all,

tl;dr, I'm running pgAdmin on a nonstandard ssl port and it breaks after
first use.

Long version: I am currently running pgAdmin4 in server mode using the
standard Apache configuration included with Debian 11 (installed via the
pgAdmin instructions, pgadmin4-web and pgadmin4-server packages
installed). The apache instance serves pgadmin over ssl on port 8443
(running a different tool on port 443) and we have a cloudflare reverse
proxy in front of that that proxies on port 443 for a particular subdomain
to port 8443 on our backend server. The first time I go to
https://example.com/pgadmin4 and log in, pgAdmin loads as expected.
However, if I log out and try to log back in, I briefly receive the pgAdmin
loading animation followed by a blank white screen rather than the
browser. If I watch the network tab of Chrome, I see 401 errors on the
following requests:
- pgadmin4/preferences/get_all
- pgadmin4/browser/check_corrupted_db_file
- pgadmin4/misc/bgprocess/

Looking at the logs, I see the 401 errors being generated in the Apache
logs on my backend server. Restarting the web server has no effect. If I
then replace https://example.com/pgadmin4 with
https://example.com:8443/pgadmin4 (ie I add the port of my Apache TLS port
rather than the expected 443 that the Cloudflare reverse proxy expects) in
my browser, pgAdmin will load again and work as expected. Due to the
security limitations of our organization, I cannot directly connect to the
backend VM on port 8443, only through the Cloudflare reverse proxy.

This seems like a bug with pgAdmin, but I'm wondering if anyone knows
whether or not I missed a configuration option that would solve this.

My Apache config is as follows:

<VirtualHost *:8443>
> SSLEngine on
> SSLCertificateFile /secrets/pgadmin-cert.pem
> SSLCertificateKeyFile /secrets/pgadmin-key.pem
>
> # enable HTTP/2, if available
> Protocols h2 http/1.1
> </VirtualHost>
>
> # modern configuration
> SSLProtocol -all +TLSv1.3
> SSLOpenSSLConfCmd Curves X25519:prime256v1:secp384r1
> SSLHonorCipherOrder off
> SSLSessionTickets off

Apache pgAdmin config

WSGIDaemonProcess pgadmin processes=1 threads=25
> python-home=/usr/pgadmin4/venv
> WSGIScriptAlias /pgadmin4 /usr/pgadmin4/web/pgAdmin4.wsgi
>
> <Directory /usr/pgadmin4/web/>
> WSGIProcessGroup pgadmin
> WSGIApplicationGroup %{GLOBAL}
> Require all granted
> </Directory>

Any ideas?

Thanks
Eamon