Azure AD Connect Error Adding second active directory domain

As subject says, we are attempting to add a second active directory domain to our existing instance of AADC for the purpose of synchronization to Azure.

The account I'm using to connect to the on-prem AD has enterprise admin rights, all relevant firewall ports are open, and i can ping both the netbios and fully qualified name.  Further, I can bind to the target domain with LDP.exe and the credentials i'm utilizing, so i know those are good....which makes the error I receive that much more confusing:

"The user name or password is incorrect.  using credentials with a fully qualified domain may help to resolve this issue."

I've attempted both with <netbios>\username as well as <domain.com>\username, result is the same.  No issues connecting to the original domain that we are synchronizing, it's just this one.

Anything else I can try?  I'm at my wit's end and my change window is closing soon...