Fatal error: Call to undefined function menu_tag_menu_item_form_submit() in /includes/form.inc on line 1460

I was able to add a menu tag to a menu item in a menu item edit form without the error.

I was able to add a menu tag to a menu item in the menu edit form without the error.

Haven't yet tested to see if it all works but THANKS! Now to go back and shift tons of menu items from individual menus into the main menu structure and switch to using Menu Tag as it clearly has significant purpose and a place in Drupal moving forward.

I was doing fine on a number of items without error, but then when trying to alter the location of a menu item to get it to show up under my "Home" (/) menu item I got this and when going back and trying to instead put it under the panel page I have functioning as "Home" (/home) I got the same thing:

Fatal error: Call to undefined function menu_tag_menu_item_form_submit() in /home/account/public_html/drupal/includes/form.inc on line 1460
PathInfo: admin/structure/menu/item/9773/edit is served by menu_edit_item (form definition), implemented by menu in /home/account/public_html/drupal/modules/menu/menu.admin.inc at line 258 with complex arguments
... (Array, 3 elements)
Krumo version 0.2.1a | http://krumo.sourceforge.net Called from /home/account/public_html/drupal/sites/all/modules/pathinfo/pathinfo.theme.inc, line 126

Hmmmmmmrmm...

Going to try flushing cache a few times and see if it's something on my end as I didn't yet since running the update.

Nope. After flushing cache twice, the WSOD and error description happened again. When returning to the menu item edit page, the settings are indeed set, so it must be something after the data is written...