If you've performed the upgrade from Windows 7 to Windows 10 (not reported for Windows 8.1 to Windows 10 upgrades), you may be experiencing an issue where Office 2013 documents will error when you try to open them. Apparently, this problem has been reported enough that Microsoft is investigating a proper fix.
A new KB article (KB3086786) has been created to acknowledge the problem and provide a couple workarounds. One workaround involves correct permission on folders, the other is to run an online repair of Office 2103. The repair is probably the easiest to do for most.
Another workaround has been reported where the problem can also be fixed by disabling or bypassing Protected View in Office applications. Microsoft warns against doing this as it could leave your computer open to attack.
We'll follow up as the situation progresses.