Dynamics365 - SQL Reporting working / FetchXML Reporting not working (rsProcessingAborted)

There are thousands of posts about the rsProcessingAborted error but most of them are pointing to general reporting (rdl) issues.

In the current project (ongoing for few years), we introduced the first FetchXML based report now. The previous ones have been SQL based (OnPrem).

After deploying the report from Test to QA, the following error code appeared.

The report cannot be displayed. (rsProcessingAborted)

Since SQL based reports were working fine it could not be a general reporting issue.
Also a newly created temp report with only one column was not working so also an error with the newly designed report could not be the reason.

After checking the SSRS logs (\Reporting Services\LogFiles), I could see following message.

Could not connect to net.tcp:///CrmSandboxSdkListener-w3wp.

After checking the Implementation Guide - "Dynamics365_PlanandDeploy.pdf", I found the following entry.

After opening the TCP 808 port, the rendering worked as expected.

cheers