In the migration environment, anything can happen.
In this case, if the permissions were correct, the error below does not appear.
To solve this issue, lets check in DFS > Namespace > Namespace Servers > right click in folder and then Share Permissions.
Check if the Everyone group is enable for change permissions.
After it, Skype for Business Server will do all the necessary adjusts and after few minutes, check again….
In my lab, this solution solved the follow errors:
21046 – Address Book Server has encountered an unexpected exception.
32050 – A Create Conference request sent to an Mcu was rejected. It will be retried but if this error continues to occur conferencing functionality will be affected.
41027 – The content file store is unavailable or full.