19.5.711 Windows

When I right click-Save to save out a MaterialX file I get a script error

Child name is not unique: surface

step 1 of troubleshooting is always try running it on the latest daily build.

Providing a hip file whould also be helpfull.

To be honest, I would say the .mtlx writing support is not really robust enough to be usable. I'd be curious if anyone else has experience to the contrary. From the docs: https://www.sidefx.com/docs/houdini/solaris/materialx.html [www.sidefx.com]

Solaris and Karma utilze MaterialX through UsdMaterialX, a plugin that allows USD to import MaterialX shaders. Houdini lets you build MaterialX materials in VOPs and translate them into a UsdShade prims automatically. While there is a script that can convert VOP nodes into .mtlx files on disk, the primary workflow Solaris supports, is MaterialX nodes encoded as USD prims. Solaris can reference MaterialX looks defined in .mtlx files on disk, but support for writing to .mtlx files is much more limited.

If you need to export .mtlx you could look into authoring MaterialX in the standalone QuiltiX editor: https://prism-pipeline.com/quiltix/ [prism-pipeline.com]. Apparently you can even use Karma as a Hydra delegate.