Code:
net.sourceforge.kolmafia.swingui.ItemManageFrame could not be loaded
This can happen if we throw an exception while the frame is being instantiated. In my experience, you do not get a DEBUG log when that happens. What DOES happen is that a stack trace is printed on my terminal window.
For a Mac, it is easy to do that: open Terminal and execute the .jar file from it via "java -jar KoLmafia-16.8.jar", for example. Obviously, you have to have your shell's paths configured to be able to find java...
I don't use Linux, but I expect it is very similar there, since both Linux and OS X are Unix systems and run basically the same shells and command line tools.
If you are on Windows, I have no advice for you. Never used it, never will.
But, if you can reproduce this and get a stack trace, I can diagnose the problem. Otherwise, there is nothing that I (or anyone) can do to help you.