When your User Stories page won't load or gives "Whoops, we hit a snag" error, it is usually due to a corrupt filter setting which prevents the board from loading. This KB will also help when you are not able to add/change/remove filters or add/change remove fields from the page. Filters are saved as a Preference on a per-user basis. To clear the potentially corrupt filter setting, you can delete the filter preference. Each user experiencing this issue will need to walk through the process to clear their own preference.
See this video for a walkthrough on adding a custom app
Error while loading Plan - User story page. Preferences issue. Preference.