Here is the link, if anyone wants to take the risk: Sandisk Ultra Plus X2316RL firmware (This FW is only for the 256GB Ultra+ SSD. It cannot be applied to capacities other than 256GB)
Sadly if have just an iso version for burning on a CD.
To be clear: The firmware was withdrawed by Sandisk. The reason is unclear!
For me the firmware still works without problems, but this could not be true for other computers!
