Apparently Customer ID should NOT have the underscore character in it. I removed all non-alphanumeric characters from both the Customer ID and password and did the entire xml import over and things are back to normal. I didn't see such stipulations in documentation.
Yeah, the customer ID should only have numbers in it.
If you're using MWG in hybrid mode, we would have set the customer ID for you (because you'd get the XML from us).
Ah, I see. I found that out the hard way, unfortunately.