I think it might be a dupe, but:

current trunk (5.1) on WIN XP

1) create feedreader sample
2) try to expand node FeedReader|Imporatnt files|XML Layer
3) exception, and another attempt expands only to Please wait.

java.lang.ClassCastException:
com.sun.org.apache.xerces.internal.parsers.XIncludeAwareParserConfiguration
cannot be cast to org.apache.xerces.xni.parser.XMLParserConfiguration
	at org.apache.xerces.parsers.SAXParser.<init>(Unknown Source)
	at org.apache.xerces.parsers.SAXParser.<init>(Unknown Source)
	at org.netbeans.tax.io.XNIBuilder$XMLBuilder.<init>(XNIBuilder.java:289)
	at org.netbeans.tax.io.XNIBuilder.buildDocument(XNIBuilder.java:111)
	at
org.netbeans.modules.xml.tax.parser.XMLParsingSupport.parse(XMLParsingSupport.java:40)
	at
org.netbeans.modules.apisupport.project.layers.LayerUtils$CookieImpl.openDocumentRoot(LayerUtils.java:286)
	at
org.netbeans.modules.apisupport.project.layers.WritableXMLFileSystem.<init>(WritableXMLFileSystem.java:117)
	at
org.netbeans.modules.apisupport.project.layers.LayerUtils$LayerHandle.layer(LayerUtils.java:436)
	at
org.netbeans.modules.apisupport.project.layers.LayerNode$1.run(LayerNode.java:98)
	at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:493)
[catch] at
org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:926)