It's because of the absolute path. Please, do the following:
1. Open Address Book Manager, create a new address book and save the xml file to the folder or USB disc where your portable Viewer is located. The book should reside in the same folder with the rutview.exe.
2. Now select the newly created address book in the Address Book Manager and click Edit. In the path field remove the path and all slashes except only the file name itself.
3. Close the ABM. Now the path to your secondary address book is relative, and so it should open regardless of the disc letter.
I still recommend not overcomplicating the setup and use our default hosted service instead of the RU Server in this particular situation. That would be more than enough for a simple setup like this. RU Server is an advanced tool used in corporate setting and multi-Viewer environment and does require network stability and a dedicated machine with static IP address or DNS name.