The AD FS Web Agent for Windows NT token-based applications encountered a serious error. The cookies that were presented ...

The AD FS Web Agent for Windows NT token-based applications encountered a serious error. The cookies that were presented by the client could not be validated. 

This condition occurs when a client presents well-formed cookies that are not valid. If the client is known to be a valid user, this error might be caused by a transient issue. For instance, trust properties (for example, certificates) may have changed recently or revocation status may not be available from the certification authority. 

User Action 
Look for additional events in the security log that may contain more details. Consider enabling failure auditing on this Web server if auditing is not already enabled.