This class of errors is due to an error in an HTML command. The are usually two reasons that the file cannot be found. Either the filename in the URL is misspelled, or the path in the URL is incorrect. The last typically happens when saving a file in Notepad as type text document. Make sure that the filename in the path exists and that it matches the name of the file on the directory.
Many browsers complain when a URL has an embedded spaces.
If the space is in a data value passed by a URL field, the value may be truncated at the space.