Sounds like the Gnome error screen.
This probably won't work anymore - but it used to be possible to simply close this fullscreen warning window by hitting Alt+F4.
Also, since this is a VirtualBox VM, try disabling graphics (2D and 3D) acceleration (VM settings), maybe Gnome is trying to show fancy 3D effects and is failing.
Otherwise, I recommend installing the MATE desktop environment, which is basically a fork of Gnome 2. It's simple and generally better than Gnome 3 if you don't care about huge icons, minimalized guis with no functionality and visual effects and instead just want a lean desktop environment (what Gnome 2 used to be). Before logging in, switch to a text console with Ctrl+Alt+F2 and log in as root (or as you and use sudo -s to become root). then:
dnf install @MATE
This installation will take a couple of minutes. Afterwards, log out of the text console (exit) and restart your display/login manager (switch back with Ctrl+Alt+F1) or simply reboot the system. Next time you log in, there will be a menu listing the installed desktop environments. Select Mate, log in and you should be good.
By any chance are you able to log in to single mode and check error logs/system logs.