In New Book, "Hear this voice first" fails with "All providers failed" although my engine works for everything else. Why?

Before version 1.9.19 the New Book form started with Gemini as the engine for everyone, and the Voice step (step 3) comes before the engine choice (step 4). On a computer with only another engine connected (for example OpenRouter), the voice preview went to Gemini, found nothing, and gave "All providers failed". From 1.9.19 the form starts with the engine you really use: the one of your last book, or the first one connected. The Voice step says "Written with <engine> · change the engine"; the link jumps to the engine choice. With no usable engine, the message names which one is missing. If you are on an older version: go to step 4 first, choose your engine, come back to step 3 and press the preview again.