Authentication denied: Boot identity not valid for Admin Server in OAM Domain

ISSUE:

Weblogic Admin Server for oam_domain failed during startup due to invalid boot.properties files.
Unable to authenticate weblogic user while starting Admin Server.

ERROR OBSERVED:

<Notice> <Security> <BEA-090082> <Security initializing using security realm myrealm.>
<Critical> <Security> <BEA-090402> <Authentication denied for weblogic: 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.>
<Critical> <WebLogicServer> <BEA-000386> <Server subsystem failed. Reason: 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.
weblogic.security.SecurityInitializationException: Authentication denied for weblogic: 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(Unknown Source)
at weblogic.security.service.CommonSecurityServiceManagerDelegateImpl.initialize(Unknown Source)
at weblogic.security.service.SecurityServiceManager.initialize(Unknown Source)
at weblogic.security.SecurityService.start(SecurityService.java:141)
at weblogic.t3.srvr.SubsystemRequest.run(SubsystemRequest.java:64)
at weblogic.work.ExecuteThread.execute(ExecuteThread.java:201)
at weblogic.work.ExecuteThread.run(ExecuteThread.java:173)
>
<Notice> <WebLogicServer> <BEA-000365> <Server state changed to FAILED>
<Error> <WebLogicServer> <BEA-000383> <A critical service failed. The server will shut itself down>
<Notice> <WebLogicServer> <BEA-000365> <Server state changed to FORCE_SHUTTING_DOWN>
<Info> <WebLogicServer> <BEA-000236> <Stopping execute threads.>

SOLUTION 1:(This worked in my case)

1)Remove boot.properties from below location: 
$DOMAIN_HOME/servers/AdminServer/security
$ rm boot.properties;

2)Delete ldap directory from below location ( Please take backup before deleting any files)
$DOMAIN_HOME/servers/AdminServer/data
$cp -r ldap ldap_ORG
$rm -rf ldap;

3)Now start the server from the command prompt and provide the username/password used to login to the Admin Console.
$DOMAIN_HOME/bin
$./startWebLogic.sh

4)Admin Server starts up without any issues

SOLUTION 2:

1)Remove boot.properties from
$DOMAIN_HOME/servers/AdminServer/security
$ rm boot.properties;

2)Create a new boot.properties file and save it as below:
username=weblogic
password=Weblogic1

3)Delete ldap directory from below location ( Please take backup before deleting any files)
$DOMAIN_HOME/servers/AdminServer/data
$cp -r ldap ldap_ORG
$rm -rf ldap;

4)Start Admin Server
$DOMAIN_HOME/bin
$nohup ./startWebLogic.sh &
Note: Upon startup boot.properties file will be encrypted.

5)Admin Server starts up without any issues.