An active theme or plugin is probably calling
add_theme_support( ‘post-thumbnails’ ….
Try commenting out the offending line in the theme/plugin source and try again.
Sorry that I can’t be more specific but I don’t know what themes, plugins and versions you have installed, so the above proposed solution is a bit generic, but it “normally” solves that problem.
This is sometimes caused by a plugin (not a theme, a plugin) that calls add_theme_support() somewhere in its code.
A few of the custom post tutorials and examples out there shows example code that includes a call to add_theme_support(), hence a few custom post plugins will probably contain that call.
add_theme_support() might cause conflict, since it’s is actually designed (or intended) to be called from a theme’s functions.php file. (See add_theme_support docs).
Removing such plugins might (will probably) fix the problem. Also try logging out and back in, after disabling all plugins.
