Thank you, but this isn't a JEvents Error. It's a error with com_content on your site. You can't create categories in articles either.
Can you please try reinstalling Joomla! core files:
Goto: Extensions -> Joomla Updater -> Reinstall Core Files
See if that helps, failing that disable content / system plugins which you have installed until you find the culprit.
Many thanks
Tony
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