I have met the same problem as yours, and I solved the two problems(a "Mojibake error" after the "create new project error" ) by copying findstr.exe and findstr.exe.mui to not only ...\RStudio\bin as the guidance of @Alencar, but also to ...\quarto\bin and ...\R-4.2.1\bin.
After all of these directories have .exe and .exe.mui, I can successully render .qmd file and create a new project!
But because my working environment is not English, I have two findstr.exe.mui files in both en-US and 'zh-CN' directory, and I don't know if the render error is caused by it.