When logging into web client, the error "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." is raised
This was caused in one instance by a hosted solution forcing http cookies to only be accepted from their domain name
Consult the Web.config file for the default web site to see if the HttpCookie.Domain policy is set. If so either remove, or connect to the system via the specified domain name.