We are all on the same Minecraft version and they have identical devices so I am having trouble understanding why one of them is having an issue and not the other.
Well, I spoke too soon because I can only join and be joined by one of them now.
Thank you. The issue does seem to have been resolved through no action of my own. Finally, I can play with them again.
Yes, I checked and rechecked their Microsoft account permissions. I just keep getting the same nonsensical error message. There is nothing wrong as far as I can tell.
I think it is a Microsoft account issue. I'm looking into it.