It might be a number of things: file size, corrupted record(s), mis-match with Endnote reference types. You might consider pre-sorting your Sente 6 library by reference types then exporting them into multiple files (instead of one big file). Presorting may help troubleshoot if the problem lies with mis-matching of the Endnote reference types. Another way of checking is to export the Sente 6 library as an .RIS file (if there’s an option to do so) to enable checking the records and tags.