SecureAuth IdP Version affected: All
"Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster."
This is a load balancer related issue. This is typically seen when a web application (SecureAuth IdP) is in a load balanced configuration and the session persistence is set to transient.
The solution is to verify/change load balancer session persistence handling to "sticky" or "IP Persistent".