Same problem here, repeatedly getting logged off of the site, especially during games. On an aside, I noted a bug whereby I was in the middle of picking a card from my discard pile (using Harbinger), and got logged out. When I logged back in, I was only given the option of not selecting a card as opposed to using the Harbinger.
I am running a MacBook Air (11-inch, Mid 2013), with El Capitan 10.11.6
Was kicked out repeatedly on both Firefox (51.0.1 (64-bit)), and Google Chrome (Version 56.0.2924.87 (64-bit)).
I do have LastPass and uBlock Origin and Avast plugin running on both browsers.
Hope this helps you guys solve the problem