Like many others here (see below) I have found that calendar links don't work, seemingly dependent on whom their internet provider is.

http://answers.microsoft.com/en-us/windowslive/forum/mail-calendar/shared-calendar-with-details-recently-broken/b544b720-d4fc-4faf-a6e8-704c0a750766

http://answers.microsoft.com/en-us/windowslive/forum/mail-sharing/outlookcom-calendar-sharing-problem-links-dont/d3214041-436e-44de-a5e0-6e9323525b0b

http://answers.microsoft.com/en-us/windowslive/forum/mail-sharing/outlookcom-calendar-sharing-problem-links-dont/694295cc-7731-46ce-a13c-1828b0561034

Raising a new question as requested below,

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https://answers.microsoft.com/en-us/windowslive/forum/mail-sharing/outlookcom-calendar-sharing-problem-links-dont/694295cc-7731-46ce-a13c-1828b0561034