It was a bug in my JavaScript code. Xtube 0.4.1-2 should be in extras-devel soon.
user@MaemoBox:~$ /opt/usr/bin/qmltube QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed. QObject::connect: Cannot queue arguments of type 'QNetworkConfigurationPrivatePointer' (Make sure 'QNetworkConfigurationPrivatePointer' is registered using qRegisterMetaType().) file:///opt/usr/share/qmltube/qml/qmltube/VideoPlaybackView.qml:900: Unable to assign [undefined] to QString text