Well, if there are multiple files in the same directory and all the files have the same exact permissions and you can open all but one of the files it sure does limit the problems. Either the bad file is really really big and IIS/DNN is blocking it, you spelled the name wrong when you tried to get it or the file is in some way corrupt (didn't get copied over completely, etc.) You could check your server event logs to see what the actual error is. or you could copy the contents of the file to a new file in the folder (make sure it has the same permissions as all the good files) and try again.