both are 64 bit, latest build. we tried a sample spreadsheet with no formulae and got the same result. also tried exporting to 97-2003 .xls, same result. most strange. hard to believe MS would deliberately set a booby trap like this unless maybe they are willing to try bullying everybody to move to the subscription model. what happens when there is no internet? can you work offline with 365? i'm going to believe there is some way around this to be found. i have not reinstalled office yet but i can't think of much else to try at the moment. thanks.

by the way, opening small spreadsheets with Excel Mobile and copying the contents into a 2016 file seems to work. large-file copy and pastes fail. is there a limit to the size of file Mobile can handle?