Hello @hannesmergler,
no entries in log file? Hmmm… Just to make sure that you are checking proper log file (there are two log files). The one that you should look is in .metadata folder named .log. If you were looking at that one and not one in …metadata\knime location named knime then there are no entries and sry for the trouble 
I would try starting KNIME with -clean parameter (see here) and if that does work then reinstall.
Regarding versions is there something stopping you from updating KNIME to latest version?
Br,
Ivan