Tried with 10.3.3.19780585, same thing happened, which lead me to check logs again and I found
"Missing right to use ExtraConfig guestinfo.userdata.encoding."
Then I figured out that I needed
"Preserve All ExtraConfig Elements During OVF Import and Export" in my "Default rights bundle", not (only) in "Organization admin" role.
EPHEMERAL_TEMP_VM now boots (but cluster is in state Reconciling, so I'm checking it out).
