Microsoft has completed work to mitigate the issue and continues to work on addressing the root cause.

Office Online web services should now be accessible from the Office 365 app launcher.

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Previous Description (January 26, 2018 11:49):

There is currently an issue preventing access to Word/Excel/PowerPoint Online services through the Office 365 web app launcher tiles.
This issue is isolated to the Office 365 app launcher and does not affect other methods of accessing/using Office Online web apps (e.g. opening the file directly from OneDrive for Business).

Microsoft is currently working towards a solution. This notification will be updated when more information is available.