I installed Ubuntu 18.04 Beta 2 with no problem but the stable Ubuntu 18.04 LTS reboots after choosing either try Ubuntu or install Ubuntu. It happens when I try installing Ubuntu in non-UEFI mode. I am using an ASUS laptop with a Lexar USB 3 flash drive.
Edit: From boot options menu I pressed Esc button and after accepting the exiting dialog
You are leaving the graphical boot menu and starting the text mode interface. [OK] [Cancel]
The following message was printed on screen
[0.000000] [Firmware Bug]: TSC_DEADLINE disabled due to Errata; please update microcode to version: 0xb2 (or later)
and then Ubuntu booted almost without any issues.