I had a similar problem with a theme i've installed.

And adding the form key solved my problem, but i've added as a hidden to the register.phtml as follows:

File Path: app/design/frontend/default/theme_folder/template/persistent/customer/form/register.phtml

Line added after the opening <form>:

<input type="hidden" name="form_key" value="<?php echo Mage::getSingleton('core/session')->getFormKey() ?>" />

I had a similar problem with a theme i've installed.

And adding the form key solved my problem, but i have added as a hidden to the register.phtml as follows:

File Path:

app/design/frontend/default/theme_folder/template/persistent/customer/form/register.phtml

Line added after the opening <form>:

<input type="hidden" name="form_key" value="<?php echo Mage::getSingleton('core/session')->getFormKey() ?>" />