To fix this problem, you have two choices. The easier choice, if you haven’t made any significant editing changes lately, would be to replace the file that’s causing the error message with your backup copy that was working correctly. The file's name will be mentioned in the end of the message.
If a working backup copy isn’t available, you’ll need to download the file that’s causing the problem. Open the file in a text editing program that doesn’t create hidden characters; Windows Notepad is a great choice. Make sure the first characters are <? and the last characters are ?> in the file. Check for any hidden characters at the end of the file, such as spaces, by moving your cursor to the end of the file and deleting any spaces.