"end of document in prolog" error message for all sites

Hello,

The HTML Validator is returning the following message today for all 
URIs, whereas previously no such message was returned:

*/Line 1, Column 1/: end of document in prolog*

*|*<*!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.o...|*

*This error may appear when the validator receives an empty document. 
Please make sure that the document you are uploading is not empty, and 
report <./feedback.html> any discrepancy.*