Problems with Windows 2012 R2 datacenter keys
Problems with Windows 2012 R2 datacenter keys
Have you explored Phone Activation? The activation assistant might not be able to reach the server due to an internet issue.
How many cores are running under your license? The 2012R2 datacenter is also tracked by core count.
I contacted them and they mentioned a limit of five activations. I'm using the 2012 Datacenter version, not R2, and it's functioning properly.