I have just downloaded Ubuntu MATE 18.10 beta amd64 ISO via torrent file.
As it is torrent, its integrity is OK. I double checked with Check disk integrity from boot menu. It is really OK.
But it does not boot on my VirtualBox 5.1.38 (UM 16.04 LTS host) neither in Try or Install modes.
VM RAM is 1.5 Gb (tested also with 4 Gb), HDD is 12 Gb.
quiet silent to
INIT_VERBOSE=y verbose does not help. It shows black screen.
Sometimes it boots, sometimes not. But any other VMs start normally.