Insert / edit link not working

There is a thread about this but it has been closed. We are on the latest WP 3.5 and are having the same issue as in the other thread. In wp-admin, if I select some text on a page from within the editor, click the ‘instert / edit link’ button, enter the details and click ‘update’, the site refreshes and takes me to the All Posts section. I have tried on Mac: Safari, Chrome & FF, and on PC I have tried IE, Chrome & FF, but the same happens. I have tried different editors including the WP built in default editor, to no avail.

Any help would be greatly appreciated. Cheers!