I am not able to edit my XLS at web version, however it works properly on mobile devices. It always opens in 'viewing' mode. I am not collaborating with other users, and I also tried logging out from all the devices... to ensure there are no user conflict / lock issues. renamed it; downloaded a copy and reuploaded it. I also tried uninstalling Office / XLS from mobile devices, but of not help.
BTW - I had password protected my file & workbook (I remember the pwd and can still open it -- but just wanted to provide this info, in case if it helps cracking the problem)