I did a quick google search and found something interesting.
Fenris error can be caused by “the game trying to launch the game on the iGPU, which is definitely not supported.” Diablo 4 requires desktop PCs to have a dedicated GPU for its minimum requirements. Therefore, it should not be run on integrated CPU graphics.
Though this is just one of many possibilities but I figured it was worth it to post here. Be sure to setup your computer and configuration so it forces the game to use your dedicated GPU.
What are the specs specifically? if you’re just barely meeting the minimum requirements this could also cause issues, not just for D4 but any game.
This may or may not help you but I’m posting it anyway. It’s a bit dated but still might be related.
Diablo 4 Fenris Error Possible Fix - Technical Support - Diablo IV Forums (blizzard.com)