Since the parser was developed for news, science and teaching texts, and is currently used and optimized for the translation of Wikipedia articles, the most likely explanation for these performance variations is the difference between the formal and professionally transcribed political jargon of Europarl on the one hand and the creative and hastily written chat texts on the other.
at least in terms of 3rd person pronouns this can also be said of the Europarl corpus, which consists of isolated monologues, and employs a more abstract and elaborative style, which is also compounded by a high attribute/noun ration, and the high incidence of (pronoun-compensating) demonstratives.