Error when editing in Safari - draft saving seems not to work

This is with the latest version of Safari (2.0.3, 417.8) from MacOS X 10.4.4

Edit a document as normal, after a period of time this appears in a prompt:

Error converting parameters for call to DraftAjax.saveDraft: URLDecoder: Illegal hex characters in escape (%) pattern - For input string: " u"

This dialog can be dismissed and editing proceeds just fine.