I had no trouble running season 21 on my 2018 Mac mini i7, Catalina and RX580 eGPU. Unfortunately, Diablo III loads only to a black screen with an oversized arrow cursor and audio with the same hardware and Big Sur. If I disable ‘Prefer External GPU’, Diablo will load to the menu screen but with the expected performance hit. I have deleted and reinstalled Diablo III with no resolution.
Issue still present on version 22.214.171.124814.
Well, it worked for a moment… I installed a new monitor tonight (upgrading from ultra-widescreen to 2k). When I launched D3, it opened in a window and displayed properly with the eGPU enabled. When I changed the display to Fullscreen and relaunched, I was back to the black screen with oversized arrow icon. Obviously the issue is resolution-related, but I just cannot find the right combination to get it to work…