Ok, an update since this might happen to others.
After deleting that entry for NibeUplink.py in /var/spool/cron/crontabs/pi the mails indeed stopped. After reboot of the pi thát remained ok, but the Domoticz dashboard wouldn't start anymore at my PC or mobile.
For certainty I then ran the next commands at the pi:
- sudo cp domoticz.sh /etc/init.d
- sudo chmod +x /etc/init.d/domoticz.sh
- sudo update-rc.d domoticz.sh defaults
After that I checked the Domoticz starting parameters with sudo nano /etc/init.d/domoticz.sh: they still were OK (USERNAME=pi, DAEMON=/home/$USERNAME/domoticz/$NAME and DAEMON_ARGS="-daemon -www 8080").
After stopping and starting Domoticz plus once again rebooting the pi, the Domoticz dashboard showed up normally on both PC and mobile. There were only a few RED meters from the Nibe, what should mean there wasn't actual RPC-data but after some minutes that was OK too (all by itself).
So at this moment everything still works perfect again.
What exactly was the cause I don't know, but above actions finally worked for me after having received about a thousand 5-minute error-mails.
So hopefully it remains like this
Nibe F1255-6 PC heatpump, SolarEdge SE7K inverter + 16 x Solarwatt panels (in-roof 275Wp) + optimizers, EtaSunPro vacuum collector (40 tubes)