It looks like there's a mis-assignment of certificates today at Office 365.  After login, the redirect to portal.office.com reports the following error:

portal.office.com uses an invalid security certificate.

The certificate is only valid for the following names: *.bing.com, *.platform.bing.com, bing.com, ieonline.microsoft.com, *.windowssearch.com, cn.ieonline.microsoft.com, *.origin.bing.com, *.mm.bing.net, *.api.bing.com, ecn.dev.virtualearth.net, *.cn.bing.net, *.cn.bing.com, *.ssl.bing.com, *.appex.bing.com, *.platform.cn.bing.com

Hopefully they'll have this resolved quickly.  Thanks to our reader John for the heads-up on this!

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UPDATE (4pm EST)

Looks like this has been resolved - note from Microsoft:

Closure Summary: On Thursday, July 10, 2014, at approximately 3:57 AM UTC, engineers identified an issue in which some customers may have encountered intermittent certificate errors when navigating to the Office 365 Customer Portal. Investigation determined that a recent update to the environment caused impact to a limited portion of capacity which is responsible for handling site certificate authorization. Engineers reconfigured settings to correct the underlying issue which mitigated impact. The issue was successfully fixed on Thursday, July 10, 2014, at 5:54 PM UTC.

Great job guys - thanks much !

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Rob VandenBrink
Metafore