Your URL validated perfectly well in an EPUB2, if placed in an XHTML 1.1 file like this:
Code:
<a href="http://hebrewbooks.org/pdfpager.aspx?req=22413&st=&pgnum=19">Test</a>
The only way I found to reproduce the mentioned error message was something like
Code:
<a href="http:">Test</a>
where the syntax of an URI specification is invalid, because otherwise epubcheck would complain about missing files or wouldn't complain at all. It would be of great help if you could specify in greater detail how your EPUB (especially the <a> element) looks like.