Strangely, I get these total screenfreezes only when I use a portal to Munteen.
Shows the following little red error top-right for a brief moment. I then have to reboot my mac. Portalling back from sanctum works well, however moving back to Therka with a portal and then using that same portal to get back to Munteen again causes similar crash to occur.
I made a screenie of the errormessage: