My comment was too early. When I set the root or call start with an initialized root, it still doesn't load. It just uses the initialized root to work with. When I set the root via the start method I get an exception:
Caused by: one.microstream.persistence.exceptions.PersistenceException: Missing runtime type for required type handler for type: de.b4.docster.model.Archive
I put nearly the same code into code without Quarkus which works fine.