Error
weblogic.security.SecurityInitializationException: Authentication denied: Boot identity not valid; The user name and/or password from the boot identity file (boot.properties) is not valid. The boot identity may have been changed since the boot identity file was created. Please edit and update the boot identity file with the proper values of username and password. The first time the updated boot identity file is used to start the server, these new values are encrypted.
at weblogic.security.service.CommonSecurityServiceManagerDelegateImpl.doBootAuthorization(CommonSecurityServiceManagerDelegateImpl.java:941)
at weblogic.security.service.CommonSecurityServiceManagerDelegateImpl.initialize(CommonSecurityServiceManagerDelegateImpl.java:1029)
at weblogic.security.service.SecurityServiceManager.initialize(SecurityServiceManager.java:854)
at weblogic.security.SecurityService.start(SecurityService.java:141)
at weblogic.t3.srvr.SubsystemRequest.run(SubsystemRequest.java:64)
Truncated. see log file for complete stacktrace

Understand Our Setup: Weblogic is install using different user, administrator user is also a different user

Solution/Troubleshoot:
1)Edit boot.properties and put default weblogic username and password in plaintext
username=weblogic
password=Weblogic1

boot.properties should be in security path:
$DOMAIN_HOME/servers/{serverName}/security

2)Rename ldap directory from below location
$DOMAIN_HOME/servers/AdminServer/data
mv ldap ldapBAK

3)Start the server using default weblogic start script
the script should be in $DOMAIN_HOME/bin
./startWebLogic.sh

4)Admin server should be Up and Running without issue.

5)Login to admin console, check if the admin user exist or not, if not, recreate and assign to Administrator group

Reference:
http://scriptstrip.blogspot.my/2012/06/how-to-solve-denied-boot-identity-not.html
http://www.iamidm.com/2015/10/authentication-denied-boot-identity-not.html