There can be many reasons, let's start with below 3 questions:
1. Is it happening with all the application or only a few?
2. Furnish complete environment details with OS, App server, cluster/standalone, DB details.?
3. Are the application or server upgraded from previous versions.?
Go to Administration Console, click Settings > User Management > Configuration > Configure Allowed Referer URL’s. The Allowed Referer list appears at the bottom of the page.