I am also getting this error. This happens after loading the connection window, right after the last step of connecting, before it attempts to render the SFTP browsing window.
I am Running winscp 5.0.7 on Windows 7 x64, service pack 1.
I took all the steps I could think of to clean up my environment to test - closed all other running programs, tried running it as administrator, cleared out temp folder, etc.
I have .NET framework 4 Client Profile and .NET Framework 4 extended installed.
Can you send me an email, so I can send you back a debug version of WinSCP to track the problem? Please include link back to this topic in your email. Also note in this topic that you have sent the email. Thanks.
You will find my address (if you log in) in my forum profile.