Using AWS AppFlow Private SAP OData Connection with different Port then 443 not supported?

I was recently setting up an AWS AppFlow Flow to pull data from SAP and store them into an S3 data lake. As the SAP source Sytem was reachable via a dedicated VPN connection only it was required to setup a private connection and so I was following the tutorial at

https://aws.amazon.com/blogs/awsforsap/share-sap-odata-services-securely-through-aws-privatelink-and-the-amazon-appflow-sap-connector/

to make it work. As several things like a VPC, an ACM certificate and the required NLB were already in place I was able to skip those steps and start with setting up a PrivateLink endpoint service with a private DNS name dedicated for the SAP connection to be used by the AppFlow Connector. And as the default HTTPS port 443 was already in use at the NLB I just picked another one that was available and added a new listener for that port having attached a target group pointing to the on-premise SAP instace's IP address. But unfortunately this setup didn't work and setting up the Connector using this Private Link Service Endpoint's DNS name and the respective NLB's port for the SAP instance always failed with error message

"Error while communicating to connector: Client error: The request to SAP failed with the status code: 400 and error message: Call to SAP endpoint timed out for GET http request".

After hours of debugging, research, documentation lookup and somehow trying to find or at least getting an idea for the mistake in our setup a colleague of mine came up with the desperate proposal to setup a new NLB and again setup everything from scratch using this NLB and use exactly the values outlined in the tutorial - and it worked! After a bit of testing it turned out that initially using a different port then 443 for the Connector was the root cause of our problem. Was anyone else already able to make such a connection work using a different port than 443? As in the Connector setup the port selection is not limited (via any kind of validation rule nor is it mentioned at any place of the official documentation - only found a hint that using HTTPS is necessary somewhere) so I can only hardly imagine that this setup is so limited to be only possible with port 443.