I have just recently started using/testing RU (Version 18.104.22.168)
I created an encrypted address book (RU Address Book.encrypted) on my work PC and stored it in my OneDrive folder (C:\Users\brent\OneDrive\Critical Software\Remote Utilities), which is shared between my work and home PC.
When I got home, I added the encrypted address book file via the Address Manager by pasting the entire file name into the file/add dialog, and added some entries to the Address Book, did some more testing and then closed Viewer. I came back later to do some more testing, and Viewer/Address Book Manager showed the address book file to be missing.
I restored the deleted file from OneDrive recycle Bin, but now every time I close Viewer (or sometimes as I update the address book file in Address Manager), the Address Book gets deleted.
I have rebooted my PC, recreated this scenario several times, each with the same auto-delete of my custom encrypted address book.
I did some further testing on this...here's what I'm seeing: It appears that, regardless of whether "Tools/Options/Protection/Encrypt address book" is ON or OFF, if I "Manage/Address book manager/Add/Local/Open existing address book file" and PASTE the file name of an ENCRYPTED address book, this file will be deleted while using the Viewer or when the Viewer is closed. That is, the Viewer "Select address book" dialog expects .XML files only, but .ENCRYPTED files can be added by pasting the entire file name, but the file will soon be deleted. So, you can create Encrypted address books on one side of a OneDrive share, but adding the encrypted address book on the other side of the Onedrive share will get the address book deleted.
Sorry for somewhat misleading previous replies. At first, we thought there was an issue with OneDrive. But the actual problem is that the program lacks the ability to load an encrypted book (a file with the .encrypted extension) to your Viewer. We also mention it in the Tips section in this documentation article.
I updated the article a bit and added a workaround, see the last item in the Tips section. In fact, if you want to use the "Dropbox" method (or OneDrive for that matter) of sharing your address book, and you want to encrypt it as well, you can simply use the Portable Viewer.
Hope that helps. Let me know if you have other questions.