Hello!
Thank you for your reply, yes, I did the update/upgrade after installation, so my mosquitto version is 1.4.10.
In the meantime I tried the quick & dirty version, downgrading mosquitto the following way: sudo apt-get install mosquitto=1.3.4-2
Then Emoncms is getting data from the configurated node. 
After that, I upgraded to the latest version again, to see if the problem could be fixed.
It took me some time to find out, that /etc/rc.local had to be modified as described in MQTT Log files
To finish, a reboot was performed, now I get the measurements, now some work has to be done to configure all my nodes.