The configuration for the default implementation of PasswordStrengthChecker in property file
jboss-as/domain-management/src/main/resources/org/jboss/as/domain/management/security/password/utility.properties
specifies an incorrect FQCN

1. class of password strenght checker. If not present, utility will revert to default implementation
   checker=org.jboss.as.domain.management.security.password.simple.Simple

The correct FQCN is
org.jboss.as.domain.management.security.password.simple.SimplePasswordStrengthChecker

Note, the fallback works, but a superfluous exception is thrown which could be avoided.
java.lang.ClassNotFoundException: org.jboss.as.domain.management.security.password.simple.Simple