Could you provide us the error message if possible?
Does this issue affect all the users in your organization?
Except for the migration, do you do other changes in your environment?
You could try to delete cached sign in credentials to fix this issue. Using Skype for Business scenario in the Microsoft Support and Recovery Assistant tool or manually clear the cached information. For more details, please refer to this article.
Moreover, use Microsoft Remote Connectivity Analyzer to test if your environment has some issues.
If no luck, it recommends you check if there is any related error message in Skype for Business server event viewer. We will do further investigation with this message.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.
Haven't received your update for a long time, any update now?
If the above suggestion helps, please be free to mark it as answer for helping more people.