I have had a very similar issue, where my synchronizations have failed twice 7/15/2018 and 8/5/2018. Each time, the logs pointed back to Defender. The difference with mine was that it automatically retried 15 minutes after the failed sync and pulled a few thousand updates down the pipe again that we had already downloaded, resulting in some massive bandwidth usage both times. After this last time, I removed Defender from the product list.

A dependency of the update was not found on the server and was not provided by the upstream server. 4c7d478b-73e5-4151-b536-b3ea1111c6ce : Revision 200

A dependency of the update was not found on the server and was not provided by the upstream server. 0d761dea-de6d-4291-913d-b59b7fa5f55e : Revision 201

A dependency of the update was not found on the server and was not provided by the upstream server. 3984a3ee-164b-49b1-bd2c-17f59f007753 : Revision 200

A dependency of the update was not found on the server and was not provided by the upstream server. 7c83de1e-272a-4325-a448-45ace593d3c9 : Revision 201

A dependency of the update was not found on the server and was not provided by the upstream server. c6aa9696-3177-4718-8253-539448b3767c : Revision 200

Error Update ID

A dependency of the update was not found on the server and was not provided by the upstream server. 2d9860e6-3597-4437-83af-b34441575fb3 : Revision 201

A dependency of the update was not found on the server and was not provided by the upstream server. 77d6fadb-b7a7-4323-8a69-8047e90f8e21 : Revision 201

A dependency of the update was not found on the server and was not provided by the upstream server. 21fb4eff-a527-4c66-b027-2355b649bb09 : Revision 201

A dependency of the update was not found on the server and was not provided by the upstream server. 555bfe22-fea2-4e2f-af80-a93cebb92dd5 : Revision 201

A dependency of the update was not found on the server and was not provided by the upstream server. 0b0e424f-a36a-45ca-9eae-499e5aec8541 : Revision 200

A dependency of the update was not found on the server and was not provided by the upstream server. 25873c15-2a01-4bd6-b1d3-dba4a8a3285c : Revision 201