I've created 25 large rock elements, and now I need to automatize the reduction, bake and export of these elements.

I tried to use the HDA Processor with the Labs GameRes node, but I am given a Bad parameter reference: "./sop_simple_baker1/LOW" error.

What is the preferred way to perform such a task using TOPs?

I've had issues in the past running certain HDA's in TOPs if the HDA has any UI related python code. The HDA processor doesn't handle that well. I'm not saying that is what is causing your issue, but it might be. I would recommend saving the Labs GameRes node as a new custom node for yourself and strip out anything you don't need, and ensure that the UI is not using any python callbacks etc.

It's sounds more complicated than it is, it will probably only take you 10-15 min to do this. From there you will likely have better results using the new node with the HDA processor.