Cloudflare Tunnel giving a cert error

After installing and enabling the Cloudflare tunnel on my openVPN Ubuntu server following all the steps on the Cloudflare docs, I get the following error when browsing the domain:
Bad gateway Error code 502

What steps can I take to fix this, where should I go have a look at?

The Tunnel shows as online and healthy, so it is running, and the CNAME record was added successfully

It seems that port 8080 is not configured correctly