Pop up registration giving Error as "Invalid form submission."

Hi Guys,

We are using Membership PRO version on an website, we recently updated the plugin to the latest version and wordpress 3.6, after that Registration form seems to not be working anymore. Whenever we tried to Register it gives error as "Invalid form submission.".

Based upon this Thread https://premium.wpmudev.org/forums/topic/invalid-form-submission
we have checked and it is sending 'nonce' as one of the parameter, checked using Firebug on Firefox.
Tried to deactivate the plugin, repaired Database (only membership tables) from phpmyadmin however still not working.

Please Advice.