Update to FlexCel 5.7 or newer. FlexCel used to use the classes in System.IO.Packaging to create xlsx files. When the generated xlsx file got too big (about 7mb), it would create a temporary file in the isolated storage. In version 5.7 we have rewritten the System.IO.Package classes in native C#, that doesn't use isolated storage at all, so you shouldn't get this error anymore.

As a side advantage, the newer classes are faster since they write directly to the actual file, instead of writing a temp version to the isolated storage and then copying from the isolated storage to the real file.