You may receive this error if you are trying to edit files that the HTML Editor cannot understand because they don't contain valid HTML code. You will not be able to edit these files with the HTML Editor.
If you are trying to open a file created in another web design program, such as Visual Site Designer, there is a way to work around this. If you have uploaded your work to your server, you can download the HTML files using an FTP client such as Direct FTP, or with the HTML Editor's Open From Web feature. To open a file from the web, go to File > Open From Web, and then enter the URL of the page you wish to download. This method only works one page at a time, so if you want to work with an entire Website instead of a single webpage, you may be better off downloading the files with an FTP client.