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I downloaded the archive for NB 5.0 build 200509122200 an ran it without importing 4.1 settings. I had also cleaned out the dev folder under the nebeans folder in my home directory. I started the ide and closed the welcome.html page. That's when I got the Unexpected Exception dialog. Clicking ok would close that dialog and right away bring up another one. I am attaching the messages.log file
Created attachment 24811 [details] the log file
I think it's dup of issue 64189. Please try last Q-build. *** This issue has been marked as a duplicate of 64189 ***