The problem is that IIS doesn't allow the page to be ./ when using METHOD=POST as the HTTP header, it works fine for get
Consequently the login forms, compose forms, and many other forms (including upload) don't work since
The ./ needs to be changed to ./index.php for IIS.
Even if the default document is index.phpand a HTTP 405 METHOD NOT ALLOWED error shows up.
I read some other fellow posted this a few weeks back but here are a few more details. It affects the latest SVN release.
More details:
http://roundcubeforum.net/forum/index.php?topic=1102.0