This article explains the possible reasons for unable to login to a device after RE replacement.

After RE replacement, while applying the configuration and committing, you lose access to the device.

OLD PASSWORDS:

• It is always advised to save the recent config before RE replacement. In few cases, the config pushed from backup might not be the latest one causing access being lost after RE replacement.
• Check if the local user and root password are set correctly.
• The authentication hash-keys and passwords might have been changed since the last backup.
• Please decrypt all the md5 passwords and check if they are correct or not. If not then set the correct password.

SYSTEM LOGIN CONFIG:

• Please make sure that the ssh config is present on the device under [edit system services].
• The access-related configuration could be the reason why you lose access. Remove the system login configuration under [edit system login].and try to login using root user.
  • If the step fails, then re-seat the RE and try again.
  • If it is a success, then the issue is with the system login configuration.
• Review the system login configuration.

AUTHENTICATION ORDER:

• If multiple authentication methods are present, then please check the authentication order under [edit system].
• Refer the document to understand more on authentication-order https://www.juniper.net/documentation/us/en/software/junos/user-access/topics/topic-map/junos-os-authentication-order.html
• Example- if we have the configuration authentication-order radius;
  • Try configured RADIUS authentication servers.
  • If a RADIUS server is available and authentication is accepted, grant access.
  • If a RADIUS server is available but authentication is rejected, deny access.
  • If no RADIUS servers are available, try local password authentication.
• Similarly, if you have the config authentication-order [ radius tacplus ]; and if a TACACS+ server is available but authentication is rejected, deny access.

TACACS ISSUE:

• Check the reachability till the TACACS+ server if it is fine.
• Double check the TACACS secret password.
• Check the monitor traffic matching "tcp port <port number using for tacacs>" 
• Check the TACAS server if it is receiving packets from your router.
• Reconfigure the TACACS related configuration.

Similarly, there could be RADIUS server related issues.