Would probably make more sense if it returned something other than 500.
In my logs I got
ActionView::Template::Error (Discourse::InvalidAccess)
/var/www/discourse/lib/auth/default_current_user_provider.rb:79:in `current_user'
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Added on my list to look into. 
5 Likes
Improved error message via:
https://github.com/discourse/discourse/commit/804b4f32f84626a6820433ab78e10e08774a9cbe
4 Likes