Page isn't loading when opening a file in "Excel for the web" on Safari

Hi - I'm hoping someone can help me.  We've been using dropbox for a while and it's been going swimmingly, but in the recent few months there has been a change.  When I open an excel document it used to open in 'Excel for the web' and was easily editable, but recently it's stopped loading properly.

I'm trying to open it using Safari browser (my friend who is using Microsoft software isn't having the same issue), and now instead of the file loading it looks like this, but never loads: 

Can anyone help?  I've restarted my computer and installed any updates.  

Many thanks.