You clearly have Steam issues (yup - Steam application). Close Steam. Logoff from it, reboot. Login. Try again.
If problems persist contact platon.fomichev@larian.com. I will request sampler output from you right away. In order to do so launch Activity Application - click on Divinity Original Sin (which when in Spinning wheel should be marked as red) and press 'Sample' button. Wait and save sampler output.
If you're a developer you will see who's to blame right away.
Given that we haven't touched multiplayer Mac at all in this version I am very positive it's Steam.
Keep me posted
