A new security policy turned off cookies on the users' browsers.
Explanation:
The most likely cause of the issue is that the SaaS provider is experiencing an outage with its directory service. A directory service is a software application that stores and organizes information about users and resources on a network. It also provides authentication and authorization services, allowing users to access resources based on their credentials and permissions. If the directory service is down or unreachable, users will not be able to log in to the external SaaS applications that rely on it. This can happen due to network failures, hardware failures, software errors, cyberattacks, or maintenance activities. The other options are less likely to cause the issue, because they would affect either the internal or the external applications, but not both. For example, if the MFA on the internal company authentication service is misconfigured, users would not be able to access the internal resources, but they could still access the external SaaS applications. Similarly, if the SSL certificate on the external SaaS application is expired, users would see a warning message or an error message, but they would not be redirected back to the log-in page. If a new company policy requires all users to reset passwords, users would be prompted to do so when they try to log in, but they would not be prevented from accessing the applications. If a new security policy turned off cookies on the users' browsers, users might experience some issues with session management or personalization, but they would not be unable to log in to the applications. References: Directory service, Troubleshooting SaaS Application Access Issues, CompTIA Cloud+ Study Guide: Exam CV0-003, Chapter 8: Cloud Security, p. 331.