I'm having the same problem with my 55" Samsung LED 3D tv. We bought this tv approximately a year ago from our local Sams Club. I will try to get the model number and software versions later. As others have stated I have to keep signing in/re-authorizing my account through the youtube.com/activate page in order to access my account on the tv. My screen also doesn't have the settings option on the main screen. I have to do something similar, in order for the youtube remote app to work as well. I have noticed that the software seems to change it's functionality with various "start ups" of the tv. Sometime the search function will have the next letter predict working, and other times it does not. Sometimes the option for connecting to a remote device is visible, and sometimes it is not.
This is most frustrating because the app worked fine up until a month or two ago. We enjoy having You Tube available on the tv, but the way the app is working now, it's almost enough to make us not want to bother with it.