kdenlive 0.7.8 has been fully installed and is up to date, including all the dependencies.
x11 is installed and updated and running normally.

when starting up kdenlive (no matter if from terminal, spotlight or applications) the icon appears in the dock, but the application won't start. it has to be terminated using 'force quit' and there's no alert. compared to other applications (like gimp, for example) there's no x11 icon appearing.
could that be the cause and what could be done about it?

kdenlive does not use X11 on Mac. Not sure what I can tell you that I have already documented in the instructions on the site here. Kdenlive on MacPorts is very difficult to support.