Problem:

A SAP transport fails with error

Unable to get meta data for IDoc type xxx with extension xxx: RELATION_NOT_FOUND -> RELATION_NOT_FOUND

Solution:

The relation between Idoc type and extension is not released to the SAP repository.
A release must be performed on SAP side by SAP admins.

Goto WE30 or WE31: In this example you can see a Z segment, with the release flag marked. This will notify the SAP repository to update itself.

Afterwards restart ET-Connector to catch the repository update.