Symfony 5.x - An error occurred while loading the web debug toolbar

This is a bug in Symfony 5.0.0 that should already be fixed in 5.0.1. You can see the configuration change, that probably will fix this here: https://github.com/symfony/symfony/compare/v5.0.0...v5.0.1#diff-414443648aa78565f2158b8154a1ff8e

The best approach would be to run composer update