Got similar problems, but permissions workaround has not helped me.
Can't sign in (not that I want to). When try to create new world (vanilla settings), immediately says "There was a problem loading this world".
FYI I think the resource pack seems to download ok.
Kid has been using Minecraft on old Fire 8in for 2 years no problem. That tablet has now died!
This happens on his new Fire10 Tablet. Works ok on this tablet in my adult profile, but not in any kids profile, including brand new ones with no knowledge of any previous settings.
Wasted 1hr+ of my life with Amazon support - just reboots and resets. Hoping the real experts can help us here!
Update: I have now got it up and running using the permissions fix - after I had read it properly and changed the permissions in the kids profiles, not just the adults!
I agree that something should still be done about this as it is very obscure - I have spent several hours researching this today and very nearly gave up. There is a note about permissions on the app purchase description, but most mortal humans are unlikely to make the connection.
Thank you to the contributors above - my autistic son is now happy again.