iCloud Drive option greyed out in System Settings
iCloud Drive greyed out in the iCloud System Settings on Mac Pro (Late 2013) works on every other device. Tried logging out of iCloud and logging back in but iCloud Drive is still greyed out. Connected to OS X Server but not an issue for Macbook Pro Retina which is able to access iCloud Drive via Finder under the same network user account.
Any help greatly appreciated.
Mac Pro (Late 2013) specs:
Yosemite 10.10.3
3.5 GHz 6-Core Intel Xeon E5
12 GB RAM
Mac Pro, OS X Yosemite (10.10.3)