Solved this by deactivating all plugins (yes, all). Re-activated the plugins one by one - there were no problematic plugin(s).
If WP crashes on you like this Please read http://codex.wordpress.org/FAQ_Troubleshooting#How_to_deactivate_all_plugins_when_not_able_to_access_the_administrative_menus.3F if you need help.
Deactivating and re-activating plugins should NOT be a solution to this issue. Something else has gone wrong in 3.5.1.