Why I am being shown a 502, post changing the name servers while I cant see any hit in Apache access log?

The 104 is the IP address of the Cloudflare proxy server. This is normal.

A 502 can also be a firewall at your host that’s blocking Cloudflare. You’re not likely to see a 502/504 in your domain logs. It might show up on the master logs for your web server or your host’s firewall logs.

https://support.cloudflare.com/hc/en-us/articles/218378978-What-should-I-do-after-seeing-a-502-or-504-gateway-error-on-my-site-