If you are running version 1.00 and have problems viewing your Private Message, please apply the following fix (thanks to mini2 for pointing it out and providing the fix):
Goto AdminCP->Plugin System->Plugin Manager.
Click on Edit behind plugin '
Add Username history to postbit(legacy)'
In the Hook Location dropdown, change 'postbit_display_complete' to 'postbit_factory' (2 lines higher) and Save. All should be fixed now.
Better is to upgrade to 1.01, all known problems have been fixed.