I experienced this (my son did) on our mac El Capitain laptop. Resulted from recent update, as OP described, game wouldn't launch, stuck on Preparing. I restarted the mac, tried again, and it launches just fine now. Have y'all tried restarting?
Note: One other thing I did which may provide an additional clue or may be a red herring: I reset my son's mac account password so I could get in and tinker. Consequently KeyChain access failed, and in launching thru the new launcher, KeyChain barked a few times but never succeeded. Yet, his Mojang/Minecraft worlds were all there and that account seemed fine, so maybe it's not at all related..
Sonov Motekaitis
I experienced this (my son did) on our mac El Capitain laptop. Resulted from recent update, as OP described, game wouldn't launch, stuck on Preparing. I restarted the mac, tried again, and it launches just fine now. Have y'all tried restarting?
Note: One other thing I did which may provide an additional clue or may be a red herring: I reset my son's mac account password so I could get in and tinker. Consequently KeyChain access failed, and in launching thru the new launcher, KeyChain barked a few times but never succeeded. Yet, his Mojang/Minecraft worlds were all there and that account seemed fine, so maybe it's not at all related..