Thanks for the hint,
i was still puzzled why I should need to add the --add-opens as I (correctly) expected that the headless process would use the settings from knime.ini.
But then another of your posts pointed me to the root cause. For whatever reasons there was an empty -vmargs argument at the end of my command line and then all the VM arguments from knime.ini are ignored. 
Anyway, problem seems to be solved right now. Jenkins is happily executing my workflow! 