This time I got it when I tried to create a View Object (JDeveloper 11g release 2 - 18.104.22.168). Or rather let me specify. I tried to create a View Object of a SQL query and on validation of the query obviously it probed the data source and it was exactly then when I got an "ORA"-error moaning about the time zone.
After trying the usual Project Settings --> Run/Debug Profile --> Launch Settings and setting the Java time zone variable there, it only started working once I've put the following line in the file "jdev.conf" (located in: /Middleware/jdeveloper/jdev/bin):
AddVMOption -Duser.timezone="+2.00" (it was "+2.00" in my case... :-) )
Then after a restart of JDeveloper it was working fine!