Now I’m finally clear, this is NOT a bad hardware problem. Happened with another card of the same brand as the original one: both are SanDisk Extreme Pro 64GB.
After issuing the error message given in the original post the device reboots - and ends up in the same error (boot loop)
What exactly could be the reason for this behaviour?
Where in the boot process is that /dev/mtdblock4 mentioned? What is the device referring to?