Man, I can appreciate your pain! That said, all I can really contribute is the useless observation that “Such is the plight of FrontPage users on Non-Micros*ft servers.”
If your credentials are “known good”, and they are entered correctly, I have no idea what to tell you except what you already know … that FrontPage’s time (such as it was) is long since over, and that it is probably time to move on.
Maybe it’s for the best in the long run; solving this particular FrontPage related problem would likely only usher the next FrontPage related problem to the front of the line, perpetuating the never ending cycle of FrontPage related frustrations for you and your client.
Not much help there, to be sure, but it is an “honest” answer, and it’s the best I can do.