Hi Tony!
I enabled the maximum error reporting in Joomla! (System - Global Configuration - Server)
And the problem became clear.
In my case I use the gk_league_news theme from gavick and for some reason, the category edit vision calls some method from one plugin used by this theme called gk_typography:
PHP Fatal error: Call to undefined method JFactory::getXMLParser() in /var/www/html/joomla/plugins/editors-xtd/gk_typography/gk_typography.php on line 77, referer: //http://XX.XX.XX.XX/administrator/index.php?option=com_categories&extension=com_jevents
The problem is that I upgrade the template (gk_league_news) but didn't update the plugin that it's used by the template (gk_typography).
Very specific problem!
After all I realized that I could have seen the error in the error_log fo the apache. It was there too. =X
Just ugraded the plugin that was in a ziped package (gk_league_news_rest_files.zip) that came with the template package and everything goes fine!
Thanks a lot for the fast response!