I was trying to help a colleague by email with this. He created a back translation for his registered project, and was perplexed why he could send and receive this back translation via USB but not to the Internet. When he did send/receive to the Internet, the back translation did not appear in the list of projects to select.
We eventually figured out the issue. He had configured the project as a back translation, but instead of choosing the main project as the base, he had chosen a resource project. (Probably the model text they used for the interlinearizer). Since resource projects do not need to be sent and received, and back translations inherit the registration status of the base text, this back translation was in a “do not send/receive” status. Once he changed the base project to the translation project, it worked.
(In addition to the send/receive issue, picking a resource as a base would also mean none of the check boxes in the back translation would ever indicate that the back translation might need updating as the translation project text was edited).