Hi @Bernhard_Boser,
The /var/log/openhab2 folder should contain the openhab.log file (based on an apt-get installation).
To get a Karaf Console, check this section of the docs: http://docs.openhab.org/administration/console.html
For more advanced logging options, check: http://docs.openhab.org/administration/logging.html & Taming openHAB 2 Logging
BR,
Dim