$toolbarItem "shortcuts" must implement interface TYPO3\CMS\Backend\Toolbar\ToolbarItemHookInterface
The following classes are affected:

  1. typo3/sysext/backend/Classes/Toolbar/ToolbarItemHookInterface
  2. typo3/sysext/backend/Classes/Toolbar/ShortcutToolbarItem
  3. typo3/sysext/backend/Classes/Toolbar/LiveSearchToolbarItem
  4. typo3/sysext/backend/Classes/Toolbar/ClearCacheToolbarItem
  5. typo3/sysext/backend/Classes/Form/Element/TreeElement.php
  6. typo3/sysext/workspaces/Classes/ExtDirect/WorkspaceSelectorToolbarItem.php
  7. typo3/sysext/opendocs/Classes/Controller/OpendocsController.php
  8. typo3/sysext/sys_action/Classes/ActionToolbarMenu.php

To solve the issue temporarily you need to modify the constructor of those classes by removing the reference operator (&) of all referenced class parameters.

// this:
public function __construct(SomeClassNameHere &$backendReference = NULL)

// becomes:
public function __construct(SomeClassNameHere $backendReference = NULL)
php file