Special characters in `template->renderObjectTemplate` method
The has been solved (kinda) - turns out CSS was being passed into the renderObjectTemplate method for an email template. This meant that Twig was (understandably) trying to process the { and } characters as objects which was causing the service to throw a wobble. 

Have stripped out the CSS for now and will see if there is another way we can pass these characters into the service without breaking it.

Many thanks @robin on the CraftCMS Slack for help identifying and debugging the problem.