Smartlock 4 pro disconnects from server

[Juergen (Jürgen Pansy)]

This topic is duplicate. For WiFi problems with Smart Lock 4th gen, please use this topic:

Poor Wifi connection Nuki 4th gen Pro

I have a Nuki 4 pro. I have established a WIFI connection with it but it is always displayed with a red signal in the app. I think the signal means that the connection is weak. I have a distance of 2 metres to the router (no wall in between). What is the reason for this?