File share witness the given key was not present in the dictionary

If you are using a HOSTS file to resolve the server name from the client, you could try making sure to use the server name only and not FQDN, per this:

http://code.msdn.microsoft.com/windowsdesktop/Hyper-V-Remote-Management-26d127c6/view/Discussions#content

Also, check the Windows Security event log to see if there are any additional details about authentication.

Hope that helps.

Dynamic quorum doesn't work by changing the quorum type, it works by modifying the NodeWeight property on one or more nodes as required.

For example, let's say I have a three node cluster running in Node Majority quorum mode. You would choose this mode as it gives the quorum an odd number of participating votes, which is required in order to make a decision.

Now let's say I lose a node. With a traditional quorum configuration, I am now running in Node Majority mode but with two votes. This means that if I lose another node without changing any of the quorum settings (either modifying the type or the NodeWeight of one of the nodes), my cluster will go down if I lose a second node.

With a dynamic quorum, the cluster will recalculate the NodeWeights on the fly. It sees that I have an even number of nodes, so it sets the NodeWeight of one of my remaining nodes to zero. Thus, that node does not effectively have a vote in the quroum, and I'm back to an odd vote count. Now if another node fails, my cluster will stay up on the remaining nodes.

You still need to select the appropriate quorum type for your configuration, dynamic quroum just works within that type to set NodeWeights as necessary to maintain cluster uptime. You still decide from the beginning if you are going to use, for example, a file share witness or a witness disk as a quorum participant. Dynamic quroum will only work if the failures have been sequential rather than simultaneous, meaning the cluster has had time to recover and recalculate between events.