Thanks for your assistance.
As it turns out the host machine was the issue here. Using another host (running Ubuntu 18.04) i was able to flash the latest version of JetPack. Only then i was able to see the actual flash log over UART.
The initial host was running Ubuntu 20.04 which did not continue the flash process as mentioned above. It just stucked at the last entry without any error / feedback both on the host system and over debug serial output.
I had a similar behavior on a windows machine (hypervisor) which was running multiple instances of Ubuntu VMs (clients), which had an USB filter for the jetson to pass it through. I was able to flash an older version of Jetpack through such a VM, but not the latest one.
Utilizing a native Ubuntu 18.04 i had no trouble whatsoever to flash a newer version. I did that on two different Jetson TX2 with the original accessories & carrier board.
Anyway, thanks for your support.