thanks for your response. As much as I hoped to receive some useful adivce on resolving the problem I have described, I thank you for your insights.
Let me first briefly explain what I have learned after a few months and then try to show you, why I would rather stay with my current setup.
Meanwhile I was luckily able to solve the problem (supposedly corrupted SD) with no major effort. For this, I simply inserted the SD card in my livelinux machine, unmounted it, inserted it in my RPi and it worked. Shutting down volumio resulted in the same problem again, so I had to repeat the insert-remove procedure and it worked again. I will try shutting down RPi via shell and see if the problem occurs.
Now, here is why I would like to stick to my setup. I used to have a (lame) router with a USB port that could serve as a remote share. I now have a better router, but I would not like to be sending large flac files over to the RPi.
I started with my storage on an old 3,5" hdd and moved to a 2,5" SSD. Not a bad solution, except I needed a USB hub (the two ports connect the Rpi to a micro WiFi donge and a USB DAC). I then thought of putting the music on the SD, which worked great with volumio 1.2b and (as far as I recall) 1.3b. The advantage is that I now no longer need the hub either and my whole system is more puristic, which I aspire.
To get a better idea of my system, I have just taken a picture of it:
It comprises the Rpi, powered by a 5.1v 2.5A power supply, a 128GB SD card and a micro wifi dongle. Above it is the USB DAC, a Musical Fidelity V90-HPA, which also features a fairly powerful head amp. I say fairly powerful, as it claims about 0.25wpc power into 32Ohms and ouput Impedance of 50Ohms. Yet, from time to time the AKG K701 (62 Ohm, max 200mA) could surely take some more loudness.