Thanks Samer. I actually do not remember to have dismissed the notification: but evidently this was the case
I finally succeeded to upgrade to 2.1.5: the right command for OS 2.1 was
sudo avrdude -c usbasp -F -p m644 -U flash:w:firmware2.1.5.hex
which gave
avrdude: warning: cannot set sck period. please check for usbasp firmware update.
avrdude: AVR device initialized and ready to accept instructions
Reading | ################################################## | 100% 0.00s
avrdude: Device signature = 0x1e9609
avrdude: NOTE: "flash" memory has been specified, an erase cycle will be performed
To disable this feature, specify the -D option.
avrdude: erasing chip
avrdude: warning: cannot set sck period. please check for usbasp firmware update.
avrdude: reading input file "firmware2.1.5.hex"
avrdude: input file firmware2.1.5.hex auto detected as Intel Hex
avrdude: writing flash (56878 bytes):
Writing | ################################################## | 100% 3.97s
avrdude: 56878 bytes of flash written
avrdude: verifying flash memory against firmware2.1.5.hex:
avrdude: load data flash data from input file firmware2.1.5.hex:
avrdude: input file firmware2.1.5.hex auto detected as Intel Hex
avrdude: input file firmware2.1.5.hex contains 56878 bytes
avrdude: reading on-chip flash data:
Reading | ################################################## | 100% 2.71s
avrdude: verifying ...
avrdude: 56878 bytes of flash verified
avrdude: safemode: Fuses OK (H:00, E:00, L:00)
avrdude: error: usbasp_transmit: error sending control message: Protocol error
avrdude done. Thank you.