I successfully recorded a JMeter script.
I added constant timer 10000 (ms), added cookies and cache threads too.
I just recorded Login and Logout steps of my AUT.
When I rerun i am getting following error with only 1 thread.
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.
But my Dev team did not accept as a defect because when I do manually it is successfully logging in and logout.
are you sending __VIEWSTATE back correctly (fully and urlencoded) to the server?
if not, a .NET app will throw an error similar to what you described.
also, verify with your devs that you need viewstate enabled on the application under test. If not, turn it off. It will be easier to test... and big viewstates can be a performance hit to the application.