Excel online broke document

Hello,

we meet a case when Excel Online brokes the document (the document is saved on Sharepoint).

We do work in parallel, with our own add-on helping us to control the behavior (it changes Excel Protection for cells and some graphical properties).

The situation looks bad:

we found that the document is broken after 6 hours from the event - we have a history with about 12 autosave document revisions. The user working on the document didn`t notice that it is broken, but other people were not able to open it. 

I did try to recover it and see, that

1. it is possible only if I have access to Sharepoint. People logged with simply Email can`t do it because they can`t open the document with URL (Excel reports about the problem)

2. even with local Excel version: Excel reports about recovery done, but when I try to open the recovered version it is still reported as broken

How we can control such a state and how how can we be warned about the problem before it is too late? what could be the cause of the document failure?

Thanks and Regards,

Andrew