When this happens to me, I just simply close the client down and then reopen it. Click the cogwheel and do a scan and repair - you might want to get yourself a drink as it takes about 10mins. Then launch the game and it should be fine. If you still get disconnected then check for server status.
Can’t help you with the iMac situation as there is a thread about it not getting love at the moment but I doubt it has to do with your situation.