I wanted to give the new shape match a try and put together the simplest of sims following this:
https://www.sidefx.com/docs/houdini/vellum/shapematching.html [www.sidefx.com]

but for some reason I cannot figure out the vellum solver doesn't work on this setup and every other frame errors out with seemingly no geo present.

Turning off "OpenCL neighbor search" in the solver fixed it for me.

The solver defaults to uniform radius, but your grain configure was using non-uniform radii. Either disable varying radii, or uncheck assume uniform radius on the solver.

Tanto
solver

sure, if you don't want any collisions.

Hi, it seems Sphere Packing can be found if you change the OpenCL Device to CPU from the Preference.
Then trying to disable "OpenCL Neighbor Search" inside Vellum Solver's Advanced Tabs works perfectly without flickering

Shape match simulations require the latest driver for NVIDIA Ampere (or later) cards. Older chipset architectures aren’t supported. If you experience problems, go to Houdini’s main menu. There, open Edit ▸ Preferences ▸ Miscellaneous and set OpenCL Device to CPU. Then, restart Houdini to apply the changes.

3. If you had to set Houdini’s OpenCL Device to CPU (see note above), go to Advanced ▸ OpenCL and check if OpenCL Neighbor Search is turned off or inactive.