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.