When executing a Nintex Workflow 2013 with a Create Appointment action, which is configured with a body field having a multiple lines text (Rich text), the following error may occur:  
Failed to invoke Microsoft Exchange web service. The 'div' start tag on line 18 position 2 does not match the end tag of 'p'. Line 18, position 88. 

The issue has been fixed on the latest version of Nintex Worklfow 3.3.1.0 
Upgrade to the latest version of Nintex Worklfow 2013.