Whenever I load up Underrail, it always starts up as a black rectangle in the center of my monitor and stays like that for a minute or two before finally loading up in fullscreen, but honestly I'm not that bothered by the wait because it does always start up eventually.
So, with that in mind, have you tried just giving it another few minutes to boot up fully?
I know that's far from a perfect fix, but going off of my own experience that's all I can really recommend. I too have Malwarebytes Anti-Malware on a Windows 10 PC, so if it works for me maybe it'll work for you, too? It seems to work for everyone that asks this question on the Steam forums (though, to be fair, they never really respond to my help, so it's hard to tell sometimes).