Until they fix Steam Login, I suggest navigating to the 64bit ESO.EXE file in the file structure and putting a shortcut to that on your Desktop. Steam users should be doing this anyways because if you launch the game from Steam you are using 32bit executable files and not getting the full benefit of a 64bit OS if you have one and limiting the System RAM usage to 4GB or less. If that don't work around the Steam issue nothing will until ZOS fixes it.
If this Login issue on the PTS is an upcoming 'feature' that we are previewing, can I just say that this should probably not make it to Live? Fix your constant login issues please, it's ridiculous that even PTS logins are broken.