Adam,
No code-behind files were created, the error references the CuteSoft_Client/CuteEditor/Dialogs/GetFileThumb.aspx page, which contains the single line:
<%@ PageLanguage="C#"Inherits="CuteEditor.Dialogs.GetFileThumb"%>
FYI, I had originally simply Xcopied the CuteSoft_Client folder into my app folder, rather than using the IDE, and was not prompted for code-behind option.
I tried in your way too, but got the same error.