In order to make co-authoring in Office 365 work effectively, follow these steps:
- Click File and go to Options.
- Open the Trust Center and click the button Trust Center Settings…
- Go to Privacy Options and make sure the box Store random numbers to improve Combine accuracy is checked.
The corresponding registry key: https://admx.help/?Category=Office2016&Policy=word16.Office.Microsoft.Policies.Windows::L_Storerandomnumbertoimprovemergeaccuracy