Strange, I just had UEFN open for a bit after writing this post, then I tried “launch session” again and suddenly it is currently working for the first time since using UEFN… I will update more if I find out what is causing it to work / not work differently. The only difference I can say right now is that I kept UEFN open, tried opening Fortnite separately (which didn’t open or start) and then I tried the same thing “launch session” button some time after that.