I've been able to run the cardhunter beta in the past few days. Today I tried to play, logged in, clicked on 'campaign' and the screen froze although I was getting birdsong sound effects. Reloading the tab produced no response. I closed the tab and ran the game again. This time the browser opened a new tab which gave the 'checking server status' message and then switched to a message saying 'Card hunter requires Flash Player, get it here:' Repeated attempts produced the same result each time. I do have the latest version of Flash Player, and other Flash games run perfectly well. I'm using Internet Explorer 10 under Windows 7 with MacAfee security software.
I have had the same issue with IE 10 starting yesterday, though I am running Windows 8. I do find the game runs fine in Chrome, however.
That message is a generic message that really means the browser failed to embed the flash object. Usually that's either because Flash Player isn't installed or the installed Flash Player is too old. It sounds like Flash Player might have crashed in your first tab, which made it stop loading entirely after that. Does it work if you completely exit and re-start your browser?
Neither of those apply - I had the latest version of Flash installed. I tried reinstalling Flash (I was having unrelated problems with another Flash game) and the first couple of times I ran Cardhunter afterwards there were no problems. I'm now back to getting the 'requires Flash Player' message. Opening the game in firefox instead of IE 10 seems to work.