While Agent try to Login the login button will disable

The client is facing a recurring event,

many times their agent’s cant login into the portal, when they try to log in they will face an issue with the login button will be disabled and 24hrs issue will be present and after 24hrs the issue will be solved automatically.

They have notified this behavior for Night shift agents and all agents have different user IDs and different creds.

This issue will not occur to any specific user id , it will happen on random agent ids

Please help me here