Hey Ray, I'm having the exact same thing happen. I remember it from a couple years back and it's all coming back to haunt me again.
Just like Orian, my DW (High Sierra/CS6) connects fine to server and separate PHP/mysql connection works. But I'm trying to update one section of a site to mysqli and the Database connection just can't be made because of that "unidentified error".
I know it's often some strange thing in the site definition but I wonder if you can see what it is cause I just can't see it even after a couple days trying various things.