Maybe someone of you will face the following trouble in Quality Center.
The error appears when you try to send defect by email . The connection test (in Site Admin) for verifying mail server connection passed successfully.

Failed to Mail Entity;
Provider null could not be instantiated: java.lang.NullPointerException;

The solution is following (was found for WebSphere and, I hope, can be adapted for other application servers):
1. Go to folder
"<WebSphere installation folder>\AppServer\java\jre\lib\"
2. Find the file
3. Uncomment the following strings and save:
javax.xml.transform.TransformerFactory=org.apache. xalan.processor.TransformerFactoryImpl
javax.xml.parsers.SAXParserFactory=org.apache.xerc es.jaxp.SAXParserFactoryImpl
javax.xml.parsers.DocumentBuilderFactory=org.apach e.xerces.jaxp.DocumentBuilderFactoryImpl
4. Restart WebSphere.

Hope it was useful.