Data Factory keeps getting "untrusted connection" error when logging into on prem SQL Server via IR on VM
OK, I fixed this by completely uninstalling then re-installing the Integration Runtime on the VM we are using as a gateway box.  This indicates that it can become corrupt, but the corruption is not visible to the user. We could still use the gateway for testing connections, but not for data transfer.  A careful look through of the logs didn't give any clue to the error that had caused the corruption.
The version I reinstalled was exactly the same as what was there previously, so it didn't appear to be an issue with the version (5.2.7695.1).