Okay... I think I found the problem. In CreateDocx.inc on line 5331 (in the transformDocument() function) there is this line.
if (!$this->_compatibilityMode) {
throw new \Exception('Running in compatibility mode. Unsupported method.');
}
Why is the "!" negation operator there? If _compatibilityMode is set to false it is going to throw an Exception that says it is running in compatibility mode. That doesn't make any sense to me.