I also had a problem where the Password "looked" like it wasn't being passed by the form. But it turned out I had misunderstood what should go in "myhost" in the following line of code of the original posted form.

<input name="_host" value="myhost" type="hidden">

The "myhost" value in the above code needs to be set to the "email" server. I had assumed that I needed to enter my RoundCube web server for that value. My email server is not the same device as my RoundCube web server. I hope this helps.

Dave