I had the same issue when trying to install 4.2. beta... but no one listen to me... better to stay away from 4.2
I already was in 4.1.6 x64. I made the upgrade to 4.2 beta and got the admin login problem. I also had no problem with the clients login in spark, but the history chat was erased.
I'm on Windows Server 2008 x64
Going to try to install 4.2 final to see what it happens.