The documentation of the plug&paint plug-in is misleading :

It says : "The argument to Q_IMPORT_PLUGIN() is the plugin's name, as specified with Q_PLUGIN_METADATA() in the plugin."

But the argument to Q_IMPORT_PLUGIN should be the class name of the main class of the plug-in instead.

Page : http://qt-project.org/doc/qt-5/qtwidgets-tools-plugandpaint-example.html#importing-static-plugins