Error loading WSDL

I'm using the SoapUI 5.2.1, and the following error message appears:

Error loading [XXXXXXXXXXXXXXXXXJavaService?wsdl]: org.apache.xmlbeans.XmlException: java.io.CharConversionException: Characters larger than 4 bytes are not supported: byte 0x92 implies a length of more than 4 bytes.

Is there any workaround that can be done?

Already tried the option enconding in the vmoptions file:
-Dfile.encoding=UTF8

But still had the problem.

Edit:

The error seems to be on the xml declaration tag. It uses ' instead of " (<?xml version='1.0' encoding='UTF-8'?>). I can't have that changed to ", so, what can i do to make the xsd importable as it is?