Cloudfront generated error for POST when everything from the server is fine

Using Cloudfront which is directed at an EC2 server, for all GET requests, everything is fine.
However POST requests, Cloudfront fails, but the correct result is coming from the server.

Header is as follows (x is blanking for privacy), notice the "Error from cloudfront" :-

General header:
Request Method: POST
Status Code: 500
Remote Address: xx.xx.xx.xx:443
Referrer Policy: strict-origin-when-cross-origin

Response header:
cache-control: no-cache, must-revalidate
content-length: 173
content-type: application/json; charset=utf-8
date: Tue, 09 Mar 2021 15:28:47 GMT
expires: Sat, 26 Jul 1997 00:00:00 GMT
server: Apache/2.4.46 () OpenSSL/1.0.2k-fips
strict-transport-security: max-age=31536000; includeSubDomains;
via: 1.1 xxxxx1ed0adxxxxxx2cc8xxxxa281f46.cloudfront.net (CloudFront)
x-amz-cf-id: ATAa-Pd296qp0ASH022eIY0Dx0x5hGBVplqWfRpCfzkrJ1rDVFEjew==
x-amz-cf-pop: DUB2-C1
x-cache: Error from cloudfront
x-content-type-options: nosniff

Why is cloudfront generating a 500 error when the origin hasn't and the origin has returned the data as it should?