Minecraft Launcher Won't Work after Microsoft Migration

Asking this wherever I can. Wondering if anyone has had a problem launching Minecraft after the Microsoft migration. On my son's iMac core2duo with El Cap, the launcher does run. When you click Microsoft Login, it's supposed to open another login window, but it doesn't. We just get a red banner that says "something went wrong". No error information is given. It works on my retina 5k, but that doesn't help him. He's been playing on this machine for many years. I believe the min. system requirement is Yosemite, so it shouldn't be that. Thanks in advance.