XML Parse Error: not well-formed

Since Update of WP to 3.9.2 i get on many (not on all) Post (new and old) this Errormessage:

XML Parse Error: not well-formed
Address: https://www.XXXX.de/wp-admin/post.php
On line 2, column 67.:
…/post.php

Where XXX is the right Domainname….

Any Idea?

https://wordpress.org/plugins/social/