imx8mp / Can't support legacy image

[Elmar Psilog]

After switching to a new RAM I am suddenly facing an issue that
Uboot/SPL stops with:

U-Boot SPL 2021.04-00020-g2b6bb90793 (Jan 25 2022 - 11:47:36 +0100)
Normal Boot
WDT:   Started with servicing (180s timeout)
Trying to boot from BOOTROM
image offset 0x0, pagesize 0x200, ivt offset 0x0
Can't support legacy image
SPL: failed to boot from all boot devices
### ERROR ### Please RESET the board ###

We running a imx8mp board where emmc is preflashed with an Uboot and
environment. Originally the board was equipped with 1GB RAM.
We switched to 2GB RAM of another supplier. The board still boots fine.
That was important, as no new preflashed emmcs are needed.
Of course (once booted) we reflash an uboot that recognizes full 2GB RAM
and have adapted training data.
Now we switched to another supplier and have this issue above.
So far I can see RAM init was successful (no errors here), so I am
wondering what might be the reason for this fail?
All I found points to some ATF stuff, but there is no change?!? What
does the RAM deal with this?

Additional weird: When booting the uboot with 2GB support (which is
2023.04) via uuu it seems not to read the environment and reports:
Loading Environment from nowhere... OK
.. although all parameters handling ENV (position, content, settings)
keep same in this newer uboot.
Ok, there is a mix of newer uboot and environment that is saved with old
uboot but that shouldn't be a problem?

For us this is a pain, as we have a lot of boards already soldered with
this preflashed emmc that doesn't boot new RAMs and has to be handled
with UUU.
I would feel better to understand the problem here...

And one important question for the future: How likely it is that switch
of RAM needs new training data? Or isn't that more a board specific
thing (if parameters like size keeps same)?

Thanks for any hints, and Merry Christmas next!