I've never used xml config files with Log4J I've always used properties files, but I had a quick look at the Log4jXmlFormat wiki and I can't see an element called category but there is one called logger which appears to have the same structure as your category elements. Are you sure the element is called category?
To be honest I hadn't realised you could even use XML config files with Log4J and they look more readable than the property files I've been using so it has given me something to look into further.
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