When you try to login to the VMware vSphere 6.0 vCenter using the C# client you recieve the following error "The server vc01.company.com could not interpret the client's request. (The remote server returned an error: (503) Server Unavailable. Call "Service.RetrieveContent" for object "ServiceInstance" on Server "dreamline".", the Web Client is also not responding.
To fix, firstly reboot the vCenter server. Either RDP to it, or login to the ESXi host that is hosting the VM and restart the guest gracefully.
Let the vCenter come back online and ensure you have left enough time for all of the services to start if you are using vCenter on a Windows Server. Once noticable improvement in vSphere 6 is the speed all of the servers come online.
Secondly, SSH to the problem ESXi hosts and issue the following command;
/sbin/services.sh restart
You should then be able to login to vCenter.