Confirmed this issue on 6.3.2.1

I've had luck with adding this as a sibling to the RTE dialog's <uiSettings> and <rtePlugins> nodes.<htmlRules jcr:primaryType="nt:unstructured">  <docType jcr:primaryType="nt:unstructured">  <typeConfig jcr:primaryType="nt:unstructured"   useSemanticMarkup="{Boolean}true">  <semanticMarkupMap   b="...

I had the same exception crop up mysteriously too. It bugged our dev server but locally everything was fine.  The problem was resolved by hitting the Refresh Packages button in /system/console/configMgr