This problem has been solved. It was caused by a USB card reader containing a 'perfect' clone of the SD card from which the pi booted, having identical UUID's for its partitions.
For more information see: How does the pi select the device to boot from?
(https://www.raspberrypi.org/forums/view ... 8&t=254754
In my case, I worked around the issue by not
using the UUID's as partition identifiers, but instead using the partition names
) as identifiers in /etc/fstab
It would also have been also possible to simply change
the UUID's of the cloned SD card. Note however, that the UUID's to be used as identifiers are the partition UUID
's, which can be displayed using: lsblk -o name,partuuid
Thanks to all participants for your replies.