The screen gamut warning shows in blue those colors that are available in the destination (sRGB in this case), but not in the gamut of your screen. Because even cheap monitors can display sRGB, checking that box should do little or nothing. The destination gamut warning shows in red those colors that are outside the destination (sRGB in this case). Checking that box can show quite a bit of red if you soft proof in sRGB, because Lightroom works with a much wider color space.
If checking the screen warning box shows a completely red image, then there is clearly something wrong. Try disabling the GPU in Preferences - Performance.