[Zubersoft]

Normally, MobileSheets will save access to the file that was selected with the "Open file in external application" smart button. However, if you uninstall/reinstall, or restore the library on a different device, it won't have permission to access that file. So you can either reselect the file in the smart button, or you can place your files under the MobileSheets storage location (Settings->Storage->Set the MobileSheets Storage Location) so that MobileSheets always has access to them without having to jump through any hoops. Putting the files under the storage location is my recommendation if that will work with your workflow.

Mike