I've seen the "Invalid Page" error when the current location/position (in the books.db on the T1) doesn't exist in the epub any more. The easiest way to trigger this was to open an unedited book on the T1, edit the book with Sigil (which usually moves the html files to folders), load the modified book on to the T1 and then try to open the book.
There's two ways to deal with this - delete the book from the T1 and re-add it, or (if you're game!) edit the books.db file with an SQLite editor and clear out the current position for the book (I don't have a books.db in front of me but let me know if you want more detailed instructions on this). I'd very seriously recommend the former under all circumstances

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Cheers,
Simon.