GET requests is ok, but POST requests returns 502

I’m running test request through service’s generated swagger ui (browser GET’s it successfully!) and test shows response and it’s headers also. Here is headers:

access-control-allow-headers: X-Requested-With, Content-Type, Origin, Cache-Control, Pragma, Authorization, Accept, Accept-Encoding
access-control-allow-methods: PUT, POST, GET, OPTIONS, DELETE
access-control-allow-origin: *
cache-control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
cf-ray: 47aa528048dc822b-KBP
content-type: text/html; charset=UTF-8
date: Fri, 16 Nov 2018 13:32:12 GMT
expect-ct: max-age=604800, report-uri=“https://report-uri.Cloudflare.com/cdn-cgi/beacon/expect-ct
expires: Thu, 01 Jan 1970 00:00:01 GMT
pragma: no-cache
server: Cloudflare
status: 502

And response body contains CF generated page with standard answer about 502 error.