Switching from html to text in the HTML editor will try to get html2text.php from the bin folder. This will result in a 404 error. On a Windows server with ASP.NET installed you can not access the bin folder.
Solution: edit editor.js (program/js)
old:
var url = rcmail.env.bin_path+'html2text.php';
new:
var url = 'program/lib/html2text.php';