When the Milestones page (Plan - Timeboxes - Milestones) won't load, it is usually due to a corrupt filter setting which prevents the board from loading. 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.
Release : SAAS
Component : AGILE CENTRAL