I'm experiencing similar problem.
I have multiple reports and one dashboard I created some months ago. It works perfectly fine on Safari and iOS App.
However - I've created a new report now and I added one tile from the report to the dashboard I usually use. Everything works fine EXCEPT the tile from the new report where I get "Unknown Error..."
This happens both in Safari AND iOS. Haven't checked on other browsers on the iPad though.
This whole thing works perfectly fine on Windows App and Android and all browsers on Windows.