VIAVI TestCenter: PX3-400G appliance unit does not boot after switching to standby version / partition

Nebula appliance was running Spirent TestCenter 5.16 version as primary and it was working fine, when doing a switch version to the secondary one (5.15), they lost connection to the appliance, and not able to connect to it using the GUI, ping or SSH. Attaching directly to the chassis a monitor a keyboard gives the customer video and the appliance asked for a login to access, however using default credentials (admin/spt_admin) didn't work.  

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