New info.
I just tested the 64GB card directly in the SD slot on the RPi (so not via the extended card reader) and then everything works fine.
In syslog:
zynthian kernel: mmc0: Command Queue Engine enabled, 31 tags
and not one CQE recovery warning.
So the problem is the unreliable SD card adapter flatcable.
@jofemodo: About 10 days ago I asked if I could get a new one because it was unreliable. I haven’t received anything yet, so could I please have one? It’s going from bad to worse.