QT_BEGIN_NAMESPACE and QT_END_NAMESPACE should not be needed in the header.

When loading, is plugin or deviceManager NULL? If plugin is NULL then the plugin DLL is either not found at all, or unable to load. If deviceManager is NULL (and plugin is not) then the loaded plugin does not contain a QObject handling the interface.
Are you certain the Device-Manager.dll is in the location identified pluginPath?
Are all of the external runtime dependencies of the plugin DLL met in the environment in which it is running? That is, if Device-Manager.dll depends on foolib.dll is that in the PATH?