In some cases, Virtual appliances (On-prem/cloud) fail to reboot after upgrade to PCS/PPS version 9.1R11.4/R11.5/R12 or later. The following is the sequence of events leading to the issue.
1. The admin initiates an upgrade to PCS/PPS version 9.1R11.4/R11.5/R12 or later from any supported upgrade path.
2. After the upgrade process, the device reboots starting to load the current OS version (9.1R11.5/9.1R12 or later).

3. However, after some time, it results in the following error and cannot recover even after it is rebooted.
Note: This issue is not known to affect Hardware devices.