Symptoms

Few users reports an 'Unauthorised process access.' error when trying to submit the request.
There are other times when the same users can access the form and submit the form without any issue. A side note, the K2 environment consists of 2 K2 servers in a load balancer.

 

Diagnoses

The issue is reproducible in below scenario.
-Generate a new IIS machine key for the K2 Smartforms Runtime site (IIS andgt K2_site andgt Runtime andgt Machine Key).
-Restart K2 Blackpearl server service, IISRESET, and clear current user's IE cache.
-Navigate to a form and you will get prompted with the error message.

When this occurs, the current session state is no longer valid and the request for a new key has sent to the second server, so essentially the user is jumping from server to server, depending how the load balancer sees fit.

According to Installation and Configuration Guideline, the machine key must be the same in all nodes in K2 farm (K2 virtue directory sub sites ? Runtime, Designer, and Viewflow).
Troubleshooting NLB Setup
http://help.k2.com/onlinehelp/k2blackpearl/icg/current/default.htm#TS/SF_Troubleshooting.htm#UsingSSL

The above link is also mentioned d parameter. When the user hits with error message, you can capture a network trace, (IE or Chrome developer tool). If the d parameter value is changing between a working session and non-working session, then the issue is associated to change in any DLL assembly files, perhaps some mismatched version of the DLL files, see this reference.
http://stackoverflow.com/questions/13634621/how-are-parameters-for-scriptresource-axd-calls-generated
 

Resolution

Updating the IIS machine keys to use the same key resolved the issue