"I used to get error message when I wanted to leave my game, now I just got it starting a session on the Bellevue Yard... ClientMaterial::SetMaterial> blocking on time-critical thread... then 23 different error messages. I'm on a MacBook Pro 2018 2.6 GHz Intel Core i7. Mojave 10.14.4"
I get those types of errors too on my test builds. Not experienced enough to know what's going on.
That error is asset-related. Whatever asset or assets that are loading up is taking a bit too long and causing a timeout to occur. I would report the error, but continuing causes no harm.