I would love to hear an answer to your question as well. I have the same problem with RedHat 8.0 When I boot up, the display just blinks back and forth between the CLI login prompt and a garbled graphics screen (presumably the graphical login screen.) This has happened to my system before. It seems to be caused by an improper shutdown (i.e. a power loss). Last time I searched for a way to resolve it, but could find none. I ended up reinstalling.
If I'm really fast, I can eventually login via the CLI (you have to type in user/password between mode switching).
Of course, I can change the init mode to CLI mode, but I'd really rather resolve the problem without reinstalling RedHat.