In case it's useful to anyone else, I had this problem was able to fix this error as follows: when trying to access Appearance, Editor –> Fatcow hosting, with installation in a /wordpress subdirectory, giving error
Sorry, can’t edit files with “..” in the name. If you are trying to edit a file in your WordPress home directory, you can just type the name of the file in.
I followed the instructions given in the following link and it worked!
Basically, you edit the file.php inside wp-admin/includes commenting out two lines. Reupload the file.php file and then you can edit the theme files inside the editor. (Prior to finding this fix, I had switched the theme out for the default twenty-eleven theme with no change in the error message; and deactivated all plugins with my lifestyle theme, also didn't help; and changed all permissions on all theme files and all containing directories to 777 which also made no difference.)
I don't really understand why this occurred, if it had something to do with the installation in a subdirectory, but this fix did work for me.
Web Developer, Sweet Design