It appears that there are some SimpleXMLElement objects in the array that is to be serialized in stack trace above, and as the error says, ‘Serialization of ‘SimpleXMLElement’ is not allowed.’ Boo.
I tried to figure out how to modify the response so it replaces all the SimpleXMLElement objects, but this was a pain. I ended up commenting out line 237 of ExactTarget#Attributes() in /gravity-forms-exacttarget/api/ExactTarget.class.php:
// set_transient('extr_attributes', $attrs, 60*60*24);
return $attrs;
I’m not sure why the $attrs need to be stored as a transient. Things seemed to work fine without it being done.
Anyway, this seems to work for me. Any other suggestions?