A load balancer may be in use and improperly configured for use with Active Roles web interfaces.
This error may potentially be caused by the web application being unable to authenticate the user while the load balancer switches application servers between tasks.
Cause and resolution is provided by Microsoft under Cause 1 and Resolution 1B in the MIcrosoft KB article:
Additional Online Guide to generate Machine Key in One Machine and copy into other servers: https://blogs.msdn.microsoft.com/amb/2012/07/31/easiest-way-to-generate-machinekey/
Please also verify that some form of cookie / session persistence is enabled within the load balancer. For more details contact your vendor.