error: Exception loading application data : JahiaXmlDocument

I am occasionally seeing the following error on a Jahia site:

Exception loading application data : JahiaXmlDocument, Exception while
loading to the
file/usr/local/jahia/tomcat/webapps/searchsf_onqlegrand/WEB-INF/web.xmlProvi
der nu.xom.xerces.XML1_0ParserConfiguration not found, root cause:Provider
nu.xom.xerces.XML1_0ParserConfiguration not found

When I first start a system, all is well.  Then this error appears -
somewhat randomly it seems, on one of the webapps.  Once it appears, it
affects multiple webapps - same error (but with a different path).

Note - that class does exist in the version of the Xom jar that I'm using.  

Thanks
Chris