Moodle 2.2.5 to 2.4 Upgrade error: switch to YUI2 to 2in3

Hi,

We run a pretty standard Moodle install, with a few 3rd party plugins and a custom theme. we've just upgraded from 2.2.5 to 2.4.1

Mostly, it works great except we are getting the following error from time to time (most notably trying to take Moodle out of Maintenance Mode!):

Coding error detected, it must be fixed by a programmer: PAGE->yui2_lib() is not available any more, use YUI 2in3 instead, see MDL-34741 for more information.

I looked at the Bug Report (https://tracker.moodle.org/browse/MDL-34741) and it says this has been fixed in 2.4 stable (which is what we installed).

I'm not sure of the next step to fix this, so any suggestions appreciated! Cheers Sam