Dear Mario,
I think, it happened because of the MSHTML.dll version mismatch. Also I am not sure which version of this control you are using but I would highly recommend you to download version 4.0.6.7 from our website from the following link:
And also please make sure that the MSHML.dll comes with this setup should be included in your project and from the properties of the reference, please set CopyLocal = true. When you ship the application to your target machine, please make sure your setup file copy both of the Editor Control dll and the MSHML.dll that came with this control.
If you still got the problem please let us know.