Hello again,
I finally (meanwhile having updated to kernel 4.10.10) figured the reason why calibre could not access the reader: an external, fully encrypted USB HDD was connected producing the following dmesg output:
Code:
[ 5532.183684] sd 8:0:0:0: Attached scsi generic sg13 type 0
[ 5532.183738] sd 8:0:0:0: [sdm] 2930272256 512-byte logical blocks: (1.50 TB/1.36 TiB)
[ 5532.184150] sd 8:0:0:0: [sdm] Test WP failed, assume Write Enabled
[ 5532.184562] sd 8:0:0:0: [sdm] Asking for cache data failed
[ 5532.184567] sd 8:0:0:0: [sdm] Assuming drive cache: write through
[ 5532.243850] sdm: sdm1
[ 5532.245721] sd 8:0:0:0: [sdm] Attached SCSI disk
[ 5532.408959] sd 8:0:0:0: [sdm] tag#0 FAILED Result: hostbyte=DID_ERROR driverbyte=DRIVER_SENSE
[ 5532.408965] sd 8:0:0:0: [sdm] tag#0 Sense Key : Hardware Error [current] [descriptor]
[ 5532.408969] sd 8:0:0:0: [sdm] tag#0 Add. Sense: No additional sense information
Mounting the encrypted drive works fine and the partition is ok as well.
Calibre will recognize and connect to the Sony PRS-650 if the USB HDD is disconnect. Calibre does also recognize the PRS-650 after mounting the encrypted partition.
So I do not know what changed with kernel > 4.5.5 which at least for me was the last confirmed working kernel with the same encrypted USB HDD connected to the computer, but somehow an unmounted encrypted USB HDD is blocking calibre from connecting to the reader properly.
Long story short: anybody encountering the same problem might solve the problem by simply disconnecting other USB mass storage devices.
Thanks again, kovidgoyal and gbm, for your patience and suggestions.
Best,
bRick