Microsoft is aware of an issue affecting a limited number of people who get an error message when installing the April 8 release of the Windows 8.1 Update.

Though Microsoft has not alluded to what the problem actually consists of, the company has provided a patch and a KB article for the specific issue.

If you receive error 0x80071a91 during installation of the update (affecting Windows RT 8.1, Windows 8.1 and Windows Server 2012 R2), make sure to apply 2939087 through Windows Update.

The KB article is here: Error 0x80071a91 when installing update 2919355 in Windows