Call to undefined function nextendimport()

Make sure both the Accordion Menu and Nextend plugins are installed and activated on your website! This error points to the Nextend plugin's code being missing.

Also don't update the Accordion Menu and Nextend plugins separately, but only together, because they are only 100% compatible with the together released versions! If you aren't sure you have the correct versions, do another update and that should assure you.