I understand the security problem, but when you have hundreds of connections, you can't remember all credentials. In this case, credentials are probably saved in clear in an unencrypted file outside of the viewer or in the viewer comment area of the connection. So, from security viewpoint it's worst.
To best overcome the security problem, maybe this feature should only be allowed when you are connected to a custom RU server or perhaps reclam RU server authentifcation request each time you want to use it (only your RU Server credentials to remember) ?
After some others tests, It seems you must restart the RU server after adressbook rights modifications ! If you do it and re-login, it seems works fine (but strange thing is if you add user explicitly, you have not to restart the RU server to changes take effect...).
Yes, "auto logon" is enabled, but, "auto logon" enabled or not, the problem still the same. Logoff / re-logon doesn't work either.
The relatable community thread indicated doesn't help me and conclusions are not clear… It seems doesn't works fine all the time (not like when user is Added explicitly).
One thing is sure, if you add explicitly the user (member of the group) in security rights of the adressbook, it works fine immediatly you valid the modification (therefore, it's not a RU server connection problem…). If you remove the user and valid the modification, logoff and re-logon, then synchronisation doesn't work again...
RU Server 184.108.40.206  Viewer 220.127.116.11 Windows 10 (1803)
If there is only group (without user from this group) autorized by security rights of an adress book, the adress book not synchronised for a user of this group. If we add autorisation for user(s) of the group explicitly, adress book synchronised well ! (see images attached to check settings).