We recently ran into this issue at a client, specifically related to Office 2013 applications.  Users would open a handful of documents or emails and then go to their taskbar to use Aero Peek to jump to one of the windows.  Pretty consistently the users would encounter a blank preview window and be unable to switch to that window.  The other windows were accessible.  Cascading the windows was the only way they could switch to the blank window.  Turns out there is a hotfix from Microsoft available for this bug.  https://support.microsoft.com/en-us/kb/2964487.   The fix is applicable to Windows 7 SP1 or Windows Server 2008 R2.