Hmm, QNAP doesn’t list Debian as supported OS: https://www.qnap.com/de-de/support/con_show.php?cid=94
Perhaps some required driver(s) is not free.
But where does boot actually stuck? It also could have something to do with UUID of the virtual drive not being important thus the bootloader looks for a drive that does not exist anymore. See: https://dietpi.com/forum/t/running-dietpi-vm-images-on-google-cloud/3090/1
In this case as mentioned in the link, install a clean Debian (if even possible, would be that best test to check whether the supported OS list is correct or Debian for whatever reason has just been forgotten) via removable drive installer an run the DietPi-PREP script on it to turn into DietPi.