After uploading the firmware files of Syslog-ng Store Box (SSB) 6 and clicked on "After reboot" the following error is displayed.
Earlier versions of SSB (< SSB 4) installed with deprecated boot loader and file system which can cause issues on SSB 6.
The upgrade is not permitted for such SSB virtual appliance installations.
Installing SSB 6 on a new VM.
Migrate SSB to a new virtual appliance.
See the following knowledge article. Link