I’ve just managed to setup a ZWave Sniffer and I’m now able to check which ZWave devices are sending the most messages over the network. I found out that one of my devices (WiDom Universal Double Switch) is sending a message every 5 seconds. This device also does regularly not react to switch messages from the controller, or it often only switches with a delay of 10 to 15 seconds. I guess this is simply explained by how much messages this device is sending.
Sadly the device does not have a report interval setting that I could adjust, so I will just replace this module with a known good module. I just wanted to report this here so that other users know that this module behaves like this.