Have you tried Opera? Another player in this thread mentioned they got it to work using that browser.

He's already mentioned trying Safari with no success. I can only assume that as nothing else is working for him, that the update he did for his MacOS is what has broken it and would likely need to be rolled back, if possible.

That is likely the issue, they have pushed an update (to your OS by the sounds of it) which has broken your ability to play. You'll need to rollback the update that did it in order to continue playing.

There is only so much that can be done when OS and Browser developers insist on doing everything they can to remove the existence of a plugin/feature. Many don't seem to quite grasp just how much it would take to remake this game in HTML5 or any other different plugin set.

In other news, they have confirmed that a new client will be coming for Windows soon, and eventually one for Mac too (no release dates given yet) so that will help alleviate most of these issues.