Commented Unassigned: Strange FileNotFoundException [9365]

I've got a worksheet with half a million rows. After writing values to cells in the first 20 rows. I call Save() on the workbook. That produces the following exception:

Could not find file 'C:\Users\nitew_000\AppData\Local\IsolatedStorage\dxiachod.3ue\sshwvvzf.hbs\Url.kxeclqde42apwq4041xfhhekihulkeam\Publisher.qolcwimjscmc2r1axln3vci10dth4a4y\Files\1vcczr3o.pnw'.

I just updated to version .74

Any ideas what file its looking for?
Comments: The error doesn't occur in the ClosedXML code so I don't think there's anything I can do. It occurs inside OpenXML SDK's file save function.