Ok, I fixed this. Say you've screwed up your netbeans installation by installing the pythonplugin then this might just work for you, provided you're using a non-windows OS. This is because Windows uses precompiled binaries to start the Netbeans IDE.
The problem that I solved is that, by default, a set of classes is not added to the Java classpath, which results in a crash. You can find whether this is also your problem by inspecting .netbeans//var/log/messages. If it displays some ClassNotFoundExceptions then we might have the same problem. If not, then at least you've got some pointers on what's going wrong and perhaps you might come up with some solution yourself. ;)
The java classpath is constructed in the file:
/<path>/<to>/<your>/<netbeans>/<installation>/platform/lib/nbexec
On OSX, this could be:
/Applications/NetBeans/NetBeans 7.1.app/Contents/Resources/NetBeans/platform/lib
In the main loop the function construct_classpath is called, which in turn calls the function build_classpath for two directories. I changed the function to this:
build_cp() {
base="$1"
append_jars_to_cp "${base}/lib/patches" "patches"
append_jars_to_cp "${base}/lib" "lib"
append_jars_to_cp "${base}/lib/locale" "locale"
# Added by me:
append_jars_to_cp "${base}/modules/ext" "ext"
}
After that modification, start your IDE and everything should work fine. Good luck!