Hello! Is your issue with an internal user (meaning, it's part of the Directory Services) or external (invited to an external email address that doesn't belong to the WorkDocs directory?
If it's internal, you can try removing the pending invitation from the Admin panel and try inviting the user again. If it's external, you should see a new user account created in Active Directory with the email address of the user as the username. You can try to manually set a password in the account and see if the user can login.
If none of the above works, please open a new support case and provide a HAR log  from the web browser while reproducing the issue and we will be able to assist further. Have a great day!
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