Users keep getting locked out of a document.
A user is able to make changes, but then everyone else isn't.
Among the errors:
"We're having trouble connecting to the server. If this keeps happening, contact your help desk."
"Upload failed. This file is locked for editing by ..."
Fiddler displays a 404 cobalt error:
Apparently, there is a key in the registry that if non-existent, will prevent co-authoring!
There is a hotfix, the October 13, 2015, update for Office 2016 (KB2920679).
Or simply add the following registry key, and then restart Word:
Windows Registry Editor Version 5.00
(this could be saved as a .reg and executed)