This KB describes NSSU failure with error message 'ISSU Aborted! Backup RE maybe in inconsistent state'.

When upgrading EX or QFX using NSSU with no-validate option, the NSSU process may fail with the following error. For example, this error actually occurred during an upgrade from version 22.4 to 23.4.

error: [May 21 11:27:58]: Backup RE not ready (GRES not Operational)
error: Backup RE not ready for ISSU
error: ISSU Aborted! Backup RE maybe in inconsistent state, Please restore backup RE
[May 21 11:27:58]:ISSU: Error
Chassis ISSU Aborted
[May 21 11:28:00]:ISSU: IDLE

The root cause was the use of unsupported configuration in the target upgrade version.
When upgrading a device using NSSU with the no-validate option, the configuration check is skipped. As a result, if the target version does not support certain configuration settings, one of members may upgrade successfully, but the NSSU process can stop partway through with error.

If the no-validate option is not used, an error will be displayed when the NSSU command is executed, and the NSSU process will not start so that you can notice unsupported config.

Validating against /config/juniper.conf.gz
/config/juniper.conf:99:(29) syntax error at 'no-tcp-forwarding'
 [edit system services ssh]
  'no-tcp-forwarding;'
   syntax error
Validation failed
error: Failed to read reply from fpc1
error: ISSU Aborted! Backup RE maybe in inconsistent state, Please restore backup RE
[May 20 19:20:33]:ISSU: Error
Chassis ISSU Aborted
[May 20 19:20:34]:ISSU: IDLE

When performing an upgrade using NSSU, please avoid using the no-validate option whenever possible.