setup:

Jenkins 2.401.3 (current LTS)

LDAP PluginVersion694.vc02a_69c9787f

Mixing Security Realm PluginVersion1.3

I have a current LDAP realm connecting to AD.
When I try to change my current LDAP realm to mixed and then adding the LDAP realm again under Mixed, I am not able to save/apply the changes in the UI, I get the gremlin.
Checking the Jenkins log I can see

Caused: java.lang.NoClassDefFoundError: hudson/util/spring/BeanBuilder

I checked internet and the best result was https://docs.cloudbees.com/docs/cloudbees-ci-kb/latest/client-and-managed-controllers/ci-2-277-1-2-acegi-to-spring-security mentioning the same NoClassDefFoundError.

Any help on this?