If upgrading to version 3,
Plugin (admin_toolbar_tools.extra_links:node.add.alerte) instance class "Drupal\admin_toolbar_tools\Plugin\Menu\MenuLinkEntity" does not exist. in Drupal\Component\Plugin\Factory\DefaultFactory::getPluginClass() (line 97 of /data-platforms/platforms/drupal9-maj/core/lib/Drupal/Component/Plugin/Factory/DefaultFactory.php).
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3.0
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
# 2 works for me.
I moved modul version with 2.x to directory contrib (mv modules/admin_toolbar modules/contrib), clear cache, update db, composer require 'drupal/admin_toolbar:^3.4', update db, clear cache.
I also removed cached files:
rm -rf sites/default/files/php
rm -rf sites/default/files/js
rm -rf sites/default/files/css