UAC. UAC is the problem. That bullshit, yes. In my case the first login was successful and working, but any other consecutive attempt was not working. I was getting either a flickering(unusable) or completely black screen, on RDP or directly on the console of the 2012 R2 server. In the Application eventlogs there were lots of errors with eventID 1000(Faulting application name: explorer.exe, version: 6.3.9600.17415, time stamp: 0x54503a3aFaulting module name: unknown). UAC was disabled. I just enabled it, reboot the server, and then I was able to log in with multiple working sessions. I even disabled back UAC and reboot the server again and things were working normally. Beware of UAC :)