Can't use "defaultbutton" in form tag in login control page.

I have a login control and I tried to set the default button as the submit button within my form tag in the login page.

But, I am getting the following error.

Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

 

I already generated the machine key.

Any clue!