You did read the release notes, right? See - https://ubuntu-mate.org/blog/ubuntu-mate-cosmic-beta/ where it says that the iso may not boot on virtualbox - This appears to impact Ubuntu (proper) more than Ubuntu MATE. It is an intermitent problem on Ubuntu MATE and simply resetting the VM may work on the next boot. If not, press a key when the initial boot prompt (Man and Keyboard) is displayed, then press F6 and select nomodeset and press ENTER to boot.
Good luck Norbert_X.