Hibernate 6.0 does not seem to be happy with this particular piece of xml in the Hibernate reverse engineering xml file:

<hibernate-reverse-engineering>
   <type-mapping>
      <sql-type jdbc-type="VARCHAR" length='20' hibernate-type="SomeUserType" />
   </type-mapping>
...
</hibernate-reverse-engineering>

The relevant section occurs in org.hibernate.tool.jdbc2cfg.OverrideBinder.TestCase.