Help troubleshooting composer upgrade

Hello. I am trying to upgrade an Elgg 2.2.2 installation by using composer (which previously was upgraded using zip files). But I the following error message:

-sh-4.2$ composer update --ignore-platform-reqs
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 0 installs, 19 updates, 0 removals     
  - Removing symfony/yaml (v3.1.4)
  - Installing symfony/yaml (v3.2.0)
    Downloading: 100%         

Script \Elgg\Composer\PostUpdate::execute handling the post-package-update event terminated with an exception

  Class Elgg\Composer\Event does not exist 

I searched around but didn't find any solution. Any idea why this error is caused?