[Always getting cloudfront error 403 when trying to post a new topic or try to reply on a topic.]

CloudFront cannot connect to origins with invalid certificates. You have a few options:

• You can configure your distribution to connect to your backed on http only.
• Put an ELB/ALB in front of your instance and terminate the TLS on the Balancer. You can use Amazon Certificate manager to create a certificate for free.
• Purchase a certificate from a 3rd party and use it on your instance.

I met this issue some time before. The request blocked by CloudFront.

Please check this list:

CDN domain added in Cloudfront Alternate Domain Names. Once you add CDN domain to Cloudfront, you can select CloudFront endpoint without typing

WAF (if any) does not block your request

Check Http and Https