"I think I know how to fix it. After I reset my Windows 11 (using 'Remove everything'), I installed Warhammer 3, then the Mod Manager. I tried using the Mod Manager first and got a message from Steam about launcher options, but I don't have a screenshot of it. After I agreed, it launched the CA launcher instead. So, I launched the game the normal way using the CA launcher, selected my settings, and then tried using the Mod Manager again, and it worked."
"Hope this can help anyone recreate this bug."