Is the server up to date on BIOS, iDrac, etc, also have you tried using the racadm racresetcfg, as outlined on Youngs link? It will cause the iDrac to reset to factory defaults, so would need to be reconfigured, but would eliminate the existing password.  
Hello, I am trying to reset an iDRAC 6 user password that was configured by a previous sysadmin and forgotten.I have been trying to change it via System Settings. It appears to take the new password, but when I reboot and attempt to log in it doesn't work. Any idea as to this cause of this issue? Thanks.
I can't reset iDRAC 6 password