Problem fixed!
Thanks to jknutter and others describing the "indefinete/libraries" -problem.
Before that I uninstalled arduino and java several times, no help.
But I had also a lot of nested "libraries" -directories in my Arduino library which I was not able to delete.
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Directory of C:\Users\xxxxxulmisxxxx\Documents\Arduino*
1.02.15 17:36 libraries
bytes in 0 files and 1 dir
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When I renamed Arduino to ArduinoBroken, I managed to launch IDE again! (And a new Arduino directory was created.)
Now 2 questions
-what creates this looong chain of libraries?! A bug in Arduino somewhere but where? Anyway, this has happened to many of us... I am surprised that there are no other solutions to this.... IT WILL HAPPEN AGAIN TO SOMEONE
-how can we clean these broken directories safely without such tricky tools?