jDownloads 3.2.57 for Joomla 3.x has a categories Save bug. It has been withdrawn. jDownloads 3.2.58 for Joomla 3.x will be available very, very soon. When it is available please update as soon as possible. For easy installation go to the Joomla Extensions Manager / Updates and install the new update. You can find details and changelog of this and other releases in the download 'details' page.