Appstream - DOMAIN_JOIN_ERROR_NOT_SUPPORTED

The DOMAIN_JOIN_ERROR_NOT_SUPPORTED error you're encountering with AppStream 2.0 is a specific domain join error that occurs when a request is not supported on a remote computer. This typically happens when a remote computer was specified in the lpServer parameter, but the call is not supported on that remote computer.

Since you mentioned that domain join works fine directly from Windows via the Image Builder but not with the fleet, there might be an issue with how the domain join is being attempted during fleet creation. Additionally, your new Image Builder stopping after pending state when using directory config suggests there could be underlying configuration issues.

Some potential troubleshooting steps:

1. Verify that the service account being used has the proper permissions to join computers to the domain.

2. Check if there are any Group Policy Objects (GPOs) that might be restricting access or causing conflicts during the domain join process.

3. Ensure that the Windows Remote Management (WS-Management) service is properly configured and set to Automatic startup.

4. If you're using any antivirus or security software, try temporarily disabling it as it might interfere with the domain join process.

5. Verify that the organizational unit (OU) distinguished name is correctly formatted and accessible.

Since you mentioned AWS Support hasn't been helpful, I would recommend submitting a new support case with detailed information about your configuration, including logs if available, and specifically mentioning the DOMAIN_JOIN_ERROR_NOT_SUPPORTED error code. This is a complex issue that might require deeper investigation of your specific environment setup.
Sources
FleetError - Amazon AppStream 2.0
Troubleshooting Notification Codes - Amazon WorkSpaces Applications
WorkSpaces Pools troubleshooting notification codes - Amazon WorkSpaces
Error in creating custom image of windows workspace | AWS re:Post