Symfony version(s) affected: 5.1

Description

After upgrading Symfony to 5.1 my CI started to fail with this error. It also occasionally happens on localhost. I'm not exactly sure why it happens yet and I'm not able to reproduce it reliably at this point. Is this bug known / resolved already?

Why is new Symfony generating a proxy for EntityManager anyway? In master branch EntityManager is in fact final so whatever is the reason this won't work with doctrine 3.0.