On the Ambari host, open /etc/ambari-server/conf/ambari.properties with a text editor and set this property

security.passwords.encryption.enabled=false

Delete /var/lib/ambari-server/keys/credentials.jceks

Delete /var/lib/ambari-server/keys/master

You must now reset the database password and, if necessary, the LDAP password. Run ambari-server setup and ambari-server setup-ldap again.