libVDA5050++ Examples Encounter MQTT Connection Issue: MqttModule not Online

[ahsnuet]

I have installed MQTT and Mosquitto successfully on my Ubuntu. But when I run built examples (handler_demo, or detailed_action_handler_demo, etc.) of the 'libVDA5050++'.

Then it causes the following errors:

[2023-12-30 16:00:07.832] [mqtt] [info] MqttModule: online
[2023-12-30 16:00:07.832] [events] [debug] Dispatching vda5050pp::core::events::ConnectionChangedEvent event with specialized ID=7
[2023-12-30 16:00:07.832] [mqtt] [debug] sendConnection(headerId=1)
[2023-12-30 16:00:07.832] [events] [error] vda5050pp::core::GenericEventManager<vda5050pp::core::events::ControlEvent> caught an exception, while processing events:
 VDA5050PPMqttError @ vda5050pp::core::messages::MqttModule::<not demanglable>(): MqttModule not online
  <no additional context>

How can I fix this?
I couldn't find the solution online, nor could find a relevant supportive community.

PS: I am unable to open an issue on their original repository, that's why I have tried here.