JCo Error in Mii 12.1.4

I am having problems with JCo in Mii 12.1.4. When the JCo Function builds the Output XML structure properly but does not build the Input Structure at all. When I look at the Log Viewer I see this error:

JCO Proxy Error: INVALID_CHARACTER_ERR: An invalid or illegal XML character is specified.

[EXCEPTION]

org.w3c.dom.DOMException: INVALID_CHARACTER_ERR: An invalid or illegal XML character is specified.

If I try to use JRA the Input and Output structures are generated correctly. In this case the same message is inserted into the log but only as a warning.

What is going on here? Can someone please help?

Thanks