I'm getting errors all the time with this node. Here's the steps:

1. create Heigthfield node
2. create Heightfield_Erode node
3. check Freeze at Frame
4. ERROR!

Both nodes have the default settings (aside from the Freeze at Frame option).

Since you have steps to reproduce the error you would get a better help by logging a bug to SideFX support https://www.sidefx.com/bugs/submit/ [www.sidefx.com]

I tested with 19.0.657 and 20.0.653 and couldn't reproduce the error on my side, so you can tell the support more about your system configuration and Houdini version, they would maybe ask for hip file where you get this error.