I have upgrading from Magento CE 1.9.0.1 to 1.9.1
And I have an error into transactional email as example:
{Error in template processing}
You have been successfully subscribed to the newsletter.
{Error in template processing}
I seems to be related to the header and footer.
I have tested to configure transactional email : System -> Configuration -> Design -> Transactional Emails -> Email Footer Template
But It is not working.
Best Answer
I have found where was my issue. The problem was into the code I had modified in app/code/local/Mage/Core/Model/Email.
I have updated these classes and It is working as expected now.