Yep, definitely looks like a bug.
Discuss it with the host, they may need to create the game again, or you may just be able to revert the turn.

Revert turn is usually the first thing to try. If it doesn't work the first time, the second time generally fixes most issues.

There is a link to the FAQ's[multiplayerrobot.com] on the GMR website which explains how to fix some common issues etc.

That is a good idea, and has already been suggested.

They are working on GMR 2.0, and have been discussing it here and taking suggestions.

Yes!!!

Average time per turn (and total turns, and any other stats) in the last 7/14/28 days would be super useful.

I like your suggestions, a lot Atlan.
And yes, it is a good system, to have all the players voting "Player X has Lost" and then either have the host remove them from the game, or have GMR do it automatically once a majority has been reached.
The rating system sounds good as well. I'm not sure about the math/weighting, and I'm too tired and covered in a cat to look at it much more now, but I definitely back your suggestion, and hope the admins are still reading this thread.

Yes and Yes to Ayu's suggestions, with one addendum:

The host should be able to choose a deputy. If the host turns on Vacation Mode, the deputy gains Host privileges until the Host returns.
If the host surrenders or gets skip kicked, the Deputy gets permanent Host privileges.
A player vote should override this system, and yes, there should definitely be a notification.

Are you an officer in any other Steam groups?
Sounds like a Steam Client bug to me. I know they have been pushing out a lot of updates recently, and most of them seem to be fixing errors caused by bug fixes or new features in previous updates.

Ahhh ok. It has happened once before, Glad to see you figured out a temporary solution.
Might you be able to show a message to first time users advising what the issue is, and that they should try again later?

After clicking "Sign in through Steam" then clicking "Yes, Sign In" the page loads for about 30 seconds and then I get a page which states "Server Error in '/' Application. Sequence contains no elements"

Screenshot: http://i1251.photobucket.com/albums/hh548/Xaviien/GMRloginerror_zps2622f302.png

It's not a huge issue, I can still play turns through the client, and I receive emails for any skips and comments etc, but there is clearly a bug somewhere.

Yes, I understand, and I didn't disagree.
That is why most people play on Prince in MP. The only difference I know of above Prince is the bonus/malus provided to the AI, so there is not much point really.