XTransformationFailureException.. or Attribute and namespace nodes cannot be added to the parent element after a text, comment, pi, or sub-element node has already been added.

I just met this fine error after a migration of BizTalk2006 maps to 2013. It is strictly connected to the use of XslCompiledTransform which is now used since BizTalk 2013.

In my case it occurred whenever an depended father field was filled with NIL.
See the example here:

Quick solution was removing the NIL value and just apply the boolean value to the father element