It's an excel file. Opening with other programs other then Excel always causes issues when big formulas are involved protected or not. I would imagine the Ipad doesn't have Excel and is trying to open with another program. While this would most likely be fine with a standard spreadsheet file it won't mesh well with a massive function file. I know from experience with files I've created.
As far as your windows 8 it is most likely you excel version. You should be running at least the 2007 version if not 2010+. Anything under that will have issues because there was a huge increase in functionality between the 2003 version and the 2007 version. This program was written with the 2013 version so that is a big reason why there may be issues with earlier versions. I can attest that it works perfect on mine which is the 2010 version.
Also pointed out by
Old Man Nathy trying to get things to still be able to sort while keeping things protected is a huge pain in the ***. But it does work perfectly as advertised with Excel 2010+.