Issues booting from eMMC

Hi

I'm using a up squared with 128GB eMMC. I successfully installed an ubuntu 18.04 (server) on the eMMC.
Now, every other restart fails (sometimes boot up is flawless, sometimes it doesn't boot):
Instead of going into the boot manager (grub), I'm asked to type in the bios password. In the BIOS setup, I then also can't see any indication that the eMMC (or its content), have been detected. After one ore more power cycles (turning up2 off and on again), it eventually boots up into Linux again. But it's totally nondeterministic.
In case it does not boot, also choosing to launch the UEFI in the CRB/Boot menu is without success.

Meanwhile I updated the firmware to the most recent version (4.0), but without any change on this behavior.

Any ideas on how to make the boot up process more reliable? is the eMMC somehow damaged?