You have a Struts web application. The error messages are configured through the applicationResoources.properties file. Whenever these properties file is changed, the error messages may now be configured differently and will result in the above unrecognizable error message. This seems to be happening in only this particular error message. Other messages are working correctly.
The following Struts configuration files may be related to the misconfiguration:
You may either include dom.jar or xml-apis.jar in your classpath.