Unable to link Mantis with Squash TM

I am trying to link Mantis to Squash TM to report issues. I have set up Mantis for a Squash project but when I try to connect to Mantis via Squash, I get this error message:

"An error occurred : ; nested exception is: org.xml.sax.SAXException: Processing instructions are not allowed within SOAP messages"

Though, the link for Mantis I have set up in the Bugtracker manager is working so I cannot really understand what the issue is.