I have an Excel workbook that I share with a number of users. I appears that when one user opened the file, all other users were cut off from it and it was no longer shared.
Is there an issue with sharing Excel workbooks between various versions of Excel?
Is it more likely something the user did when opening or using the file, like unsharing the workbook?
Are there limitations to the number of users who can share a workbook simultaneously?

I'm looking to do more workbook sharing, but I want to avoid the issues I had last time.
I will protect workbooks going forward, so users can't unshare.