Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc.

If possible, make the script source available online and provide
an URL to it here. Try to avoid embedding huge scripts into the report.

Here is the code:
<?
$document = new DOMDocument("1.0", "utf-8");
$document->load("http://www.ecb.int/stats/eurofxref/eurofxref-daily.xml");
$xPath = new DOMXPath($document);
$result = $xPath->query("/gesmes:Envelope/:Cube/:Cube/:Cube[@currency='USD']");
//Oops! $result is somehow empty
if ($result->length > 0) {
  print "OK!";
} else {
  print "ERROR!";
}
?>

Warning: DOMXPath::query(): Invalid expression in /tmp/3.php on line 5

Warning: DOMXPath::query(): Invalid expression in /tmp/3.php on line 5
bool(false)

Did you try to enable display_errors ?

I ve localized the code from my framework and it gives errors now. Seems to me that the problem is in the QUERY. But on thing is still confusing me why the QUERY works via the

http://www.activsoftware.com/xml/xpath/ processor and some others?

.