When selecting the gateway for the dataset there used to be a drop-down that let you see the connections in the dataset and it also allowed you to link any that weren't implicity matched already. The...
I'm sure you have already tried this, but I think it bears repeating. Have you double-checked that the credentials are still correct and the password has not changed? Or since it is a direct server connection maybe the white list needs to be updated?
Other than that, I really haven't a clue. I even tried searching online but that particular error code was not found.