I’ve updated the theme demo to 3.5 and it works well, thus the error you are describing may not be related to Sensei. In any case, I will try to help out – for that, could you please send me an email to email@example.com with your website’s URL as well as admin login and password, so I could troubleshoot.
I have tested the plugin and found that the “Contact Form” module within Jetpack is causing this error since it tries to re-declare the same functions as the built-in contact form. Thank you for pointing this out, I will definitely include the fix into the next theme update (due pretty soon), and in the meanwhile you can download the updated file from my repository, unzip it, and upload via FTP into the following theme folder: /wp-content/themes/Sensei/includes/contact-form/
replacing the existing file. Please let me know if it helps, and my apologies for the inconvenience!