Rooted my Asus a few days after I have received it. A few month later I unlocked the bootloader and here we go :) Made a backup of the original stock ROM with TWRP and using Goo.im I updated to latest CyanogenMod. Everything worked fine - until I restored the stock ROM using my backup.
Recovery worked fine - but after reboot to the new image I was stuck at the ASUS Splash Screen. Fastboot was working but the only options I had was USB or Wipe Data - no Android Symbol there and no Recovery.
I found this thread at XDA which unbricked my tablet. What you have to do is easy and straight forward, you have to install a new boot.blob - how?
fastboot -i 0x0b05 flash boot boot.blob
fastboot -i 0x0b05 reboot
This should grant you access to recovery again. From the recovery you are able to use ADB and can push a new ROM to your Prime.
adb push virtuous_prime_s_18.104.22.168_v1.zip /sdcard/
Thanks to the XDA member for solving this.