Here's a solution I provided to another member:
I'll be uploading an updated version of the plugin in the next few days; until then, here's a fix:
In the "accordion_menu_walker.php" file in the plugin folder, find the following code:
class Accordion_Menu_Walker extends Walker_Nav_Menu{
function display_element ($element, &$children_elements, $max_depth, $depth = 0, $args, &$output){
And change it to:
class Accordion_Menu_Walker extends Walker_Nav_Menu{
var $options;
function display_element ($element, &$children_elements, $max_depth, $depth = 0, $args, &$output){
That should fix all the errors.
If you don't want to add that single line of code, you can just grab the latest download of the plugin, as I have already updated the download with the new code.