The four plugins are:
Update (03/21/13): Many users experiencing the unknown exception issue with Notepad++ versions since 5.9 have resolved the problem by uninstalling the PreviewHTML.dll plugin instead (see comments).
Users can move the plugins one at a time into another folder within the “plugins” folder and relaunching the program to identify the plugin(s) responsible. In the case of the WinXP machine noted above with this problem, it was necessary to move only a single plugin, the NppExternalLexers.dll file, into another folder (e.g., disabled_plugins) to get Notepad++ to launch without error. The responsible plugin(s) can also be deleted or renamed.