"The category already exist" message when creating a new category in IE6

• XE 1.9 SNAPSHOT 17115 + IE6 in WinXP
• Go to the blog
• Create a new article
• Go to the Category section at the bottom
• Enter the name of a new category (say, "Plop")and click on "Add"
• A message pops up saying that "This category already exists"
• The new category is actually being created (if you go to Main.WebHome you'll see that the "Plop" document has been created)
• The blog article page is not updated with the new category (although the category has been created)

So the category is being created but IE6 thinks it is not.

UPDATE: additionnally, the page cannot be saved afterwards (clicking "Cancel", "Preview", "Save & Continue" or "Save & View" does not do anything).