looks like your boot.properties file for the Managed Server is either corrupt or it contains a wrong password for the weblogic user.
You should review the boot.properties file in $WL_DOMAIN/servers/AdminServer/security. you should have the WebLogic Admin Console credential in the follow format:
After you change it this file you try to start weblogic again
If you configured the boot.properties and it is the first time you raise the WLS, I recommend that you do a startWeblogic without &
Security directory has to be copied from admin server to managed server after starting admin server, so that admin server will encrypt the boot.properties file and managed server uses that file while starting. Please start admin server and copy boot.properties file to security directory of managed server.