I burned ventoy boot media with MBR.
Tried latest dev image mentioned in the initial post of the thread: the same error message.
I went through linked post Memory Test no longer included on live ISOs? - #2 by M0nst3r by Zesko and replaced the line to linux16 /boot/memtest.
Pressing Ctrl+X or F10 in try to save and boot it leads to PC reboot.
I went into GRUB cmd line:
It looks like it is the memtest file issue: it quits immediately after calling it, but if to install OS from the LiveCD media and do not update the OS, and to try to run Memory test in GRUB menu w/o any edition, then Memtest starts fine.