Issue unresolved on other topic with suggestion for either change callback script to http from https or set up subdomain. However simple solution that worked for me is:
https://support.cloudflare.com/hc/en-us/articles/200172316-How-do-I-include-or-exclude-a-specific-URL-from-Cloudflare-s-page-rules-
Just remove the callback script from the cache and works fine.
Saved lots of time.