You need your site set to DEVELOPER debug, and you need to be able to access the PHP error logs, since the error is reported duing an ajax request, so won't appear on-screen. Go to any course page with more than one block in a column. Turn editing on. Edit the bottom block. Set the 'Weight on this page' option to 10. Move any other block by by dragging it below the block you just edited. Verify that the block moves with no PHP notices being logged. (We were seeing [04-Mar-2015 10:57:51 Europe/London] PHP Notice: Undefined offset: 34 in /vle/www_root/lib/blocklib.php on line 1624 [04-Mar-2015 10:57:51 Europe/London] PHP Warning: Invalid argument supplied for foreach() in /vle/www_root/lib/blocklib.php on line 1624 )