If you download Stardew Valley from Fitgirl you might encounter mentioned issue. Seems like Proton-GE no longer works. If you facing issue like this just use Proton - Experimental.

Guide

  1. Open Lutris & Click the + icon (top-left)
  2. Select Add Locally installed game
  3. Go to Runner options tab
  4. Finally you’ll see wine version, Select Proton - Experimental