Looking at the supported server OS for 14.1 it list 

Windows Server 2012 R2 Standard*
Windows Server 2012 R2 Datacenter Edition*
Windows Server 2012 R2 Essentials*

Looking at 15.1 it only list

Windows Server 2012 R2 Standard*

Is support for 2012 R2 Essentials dropped in 15.1 ?