The issue might be occurring if the Viewer installation folder or the address book file was somehow damaged or corrupted (perhaps, by some antivirus software). Please try completely uninstalling Viewer as described in this full uninstallation guide. Before uninstalling Viewer completely please copy your address book from an automatic backup as described in this guide. Automatic daily backups of your address books are enabled by default when you install Viewer and are located in this folder: C:\Users\user_name\AppData\Roaming\Remote Utilities Files\backups\.
After you've uninstalled Viewer completely, please try re-installing it once again from scratch, but this time I recommend that you also add the Viewer installation folder to your antivirus/firewall software exception list as described here.