A 502 bad gateway would refer to inbound traffic. It’s unlikely, but if it is an SSL issue, you’ll need to look towards the bottom of the page you referenced for the Inbound configuration items.
More likely, as you’re upgrading from Nexus 2 is that the URL structure changed between 2 and 3. There is a bridge mode so that Nexus 3 will support the old style URLs. Refer to the “ConfiguringLegacyURLPaths” section of this page: Upgrade Procedures
Depending what your desired configuration is, you may need to make a change in Nexus, or you may need to make a change to your forwarding URLs in your Reverse Proxy, or maybe both.