Never ever run with admin rights on an account you use everyday. Anything nasty comes near you and its curtains.
Even when we log into a NIX server we never do it as root, we always go in as a normal user then su - in a shell to start work. Most servers won't even let you. Its partly for protection and also party to be able to trace who has done what when you have multiple admins tinkering.
The localdomain could be either machine to be honest as it a common default on a build.
You could change either machine to see which one is giving you the issue but I am nearly certain it will be the Microsoft one. I suspect you have ticked a box on some security setting and the reason why its asking is to allow some sync function to work.
You could try creating a share on another machine and connecting to that to see if you get the same issue. If you don't creat a new share on the NAS box and again see and use that one for testing to protect your data in the true share.