What is your IS version on?
Please check the below article and resolution:
Consumer webservice:
In 8.2 we get a weird error like below:
[ISS.0088.9135] A WMDocument Exception was thrown in the server, usually because an XML block was not well-formed.
Resolution:
Customer installed the latest fix and it resolved the issue.
IS_8.2_SP2_Core_Fix8
Other resolution is re-generate the web service connector and try it:
HTH,
RMG
#webMethods#Integration-Server-and-ESB#Flow-and-Java-services