More information is needed. It sounds like the app in question if Netscape 4.x, but I'm not certain. A screen pic of the error dialog would be good also. If the error dialog is from Netscape (as opposed to the Finder or OS), it probably refers to a file inside the Essential Files folder, rather than the folder itself.
Assuming it is Netscape, please post a directory list of the Essential Files folder, I'll tell you if any are missing that are in my Netscape folder.