Windows – Hyper-V Manager can’t connect to Windows Hyper-V Server 2012 R2 Core

A short overview:

• Server running Windows Hyper-V Server 2012 R2 Core "Adama", up to
  date
• Core is accessible via RDP Running VM with Windows Server
  Essentials 2012 "Apollo", also up to date and accessible via RDP
• Apollo functions as a File server, AD Services haven't been activated
  yet, so there is a domain, but unused, everything goes via WORKGROUP
• Client running Windows 8.1 Pro, up to date
• All via local network

My situation:

Everything used to be running well, all the mmc snap-ins were available on the client, etc, etc… The last time I used the Hyper-V Manager on the Client has been more then a month ago. Today I fired it up, wanting to add a new VM for a postgresql DB, and I'm unable to connect to the Hyper-V server with the know error "An error occurred while attempting to connect…. Check that the Virtual Machine…. authorized to connect to …."

I've been checking anything I (and google ;)) can think of, all accounts seem to be in order via cmdkey, via mmc I can connect, but not to the Disk Management (Unable to connect to Virtual Disk Service), Device Manager. The Firewall isn't accessible anymore, but the Task Scheduler, Performance, Users, etc… are still accessible…

So in short, to me it seems to be a firewall issue, but I'm not well versed in the windows firewall and all it's possibilities, if anyone has any idea/suggestions?

Thanks & regards,

Fre