Load attached file in XML Copy Editor 1.2.0.9 (Windows) and 1.2.0.6 (Ubuntu).
Hit F9 or choose XML/Evaluate XPath, enter /productcatalog:ProductCatalog/Products/products/productNumber
Result: Cannot evaluate XPath.
Note: xml document is well formed and the particular XPath is legitimate (nothing fancy).
xpath expression needs to handle namespace properly, e.g.
/*[local-name()="ProductCatalog"]/Products/products/productNumber
Thanks for the workaround, Bernhard!