This is not only an application password, perhaps the wording in the interface is misleading a bit. This is also address book encryption passphrase. If you forgot the passphrase, you cannot unlock or otherwise restore the address books because they remain encrypted.
You can, however, unlock the interface if you delete config_4.xml file located in the Viewer application folder (%APPDATA%\Remote Utilities Files\ ) . Note, that the Viewer won't be able to load your encrypted books (because they are encrypted on disc) and will create a new, empty default address book for you.
Hope that helps. You can learn more about how address book encryption works by following the link above.