Microsoft has indicated that the issue with Light mode has been resolved.

If any users are still experiencing issues with Light mode please contact the IT Support Centre

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Previous Description (July 11, 2018 14:13):

Users with Light mode enabled in Outlook Web App may be unable to open email messages.

This is a known issue and Microsoft is working on a solution.

In the meantime, user's affected by this problem can disable Light mode.

An update will be posted when this issue has been fixed.