The game communicates with the locally hosted KoboldCPP server in the same way it would with the demo server or OpenRouter.
To me it seems like the backend is failing in some way without the game noticing, and then the request errors out.
KoboldCPP outputs are currently sent to /dev/null on Linux because I had some worries about compatibility with different distros, so there's no way to see why this is happening either. Since it's model specific, maybe the download got corrupted somehow. Try deleting the Gemma-4-Sparse .gguf and .meta files in Silverpine 1.6.6b\Silverpine_Data\StreamingAssets to redownload it.
Another thing it could be is that Gemma 4 uses SWA, which might be an issue with AMD somehow?
If anything, I will look into adding better logging for the outputs on Linux in 1.7.