The issue I am facing:
I'm running Pi-hole, PiVPN, and Unbound on an ARM Compute Instance from Oracle Cloud. My client is a Windows machine behind CGNAT. When connected to PiVPN, I'm experiencing issues accessing websites that use Cloudflare DNS. On Chromium-based browsers, these websites return either ERR_SSL_PROTOCOL_ERROR or ERR_QUIC_PROTOCOL_ERROR. On Firefox and its forks, the same websites return SSL_ERROR_RX_MALFORMED_SERVER_HELLO.
Details about my system:
Server: Ubuntu 22.04.5 LTS
Client: Windows 10 22H2
What I have changed since installing Pi-hole:
I have installed Unbound and PiVPN.
Those are SSL protocol errors.
SSL negotiation only starts once DNS resolution has successfully completed, so neither DNS in general nor Pi-hole in specific would be involved here.
As often with cryptographic procedures, you may want to verify time and time zone settings on the system running the browser are correct. But as you observing errors only for certain sites, I doubt that your time is off, else you would see errors for any site.
You should consider to consult forums specialising in those browser errors, and probably for your VPN as well, as you state to observe errors only when connected to PiVPN.