After further investigation, I come to the conclusion that some bug prevents /dev/cdrom from being created in the above constellation. Later versions of Knoppix don't show the bug, and in Knoppix 3.7 the same DVD drive works perfect.
Does Knoppix 3.5 DVD - Kernel 2.6 have known problems with DVD drives? Mine is recognized under 2.4 but not under 2.6 on this DVD.
My DVD drive is:
Code:ide_info /dev/hdc MODEL="PIONEER DVD RW DVR-106D" FW_REV="1.07"
After further investigation, I come to the conclusion that some bug prevents /dev/cdrom from being created in the above constellation. Later versions of Knoppix don't show the bug, and in Knoppix 3.7 the same DVD drive works perfect.

GIGABYTE A520M DS3H AC AMD AM4 Motherboard mATX
$59.99
Corsair RMe RM850e 850W 80 Plus Gold Fully Modular Low Noise Certified Refurb
$58.99
Gigabyte GA-B85M-D3H Motherboard
$54.99
ASUS PRIME Z890-P WIFI Z890 LGA 1851 ATX motherboard, Intel® Core™ Ultra Series
$235.99
Asus Prime B660M-A AC D4 Motherboard Intel LGA 1700 14th 13th 12th Gen DDR4
$79.99
GIGABYTE B365 HD3 LGA1151 Intel 9th Gen motherboard
$59.99
Intel H81 Motherboard M.2 NVMe LGA 1150 mATX w/ IO Shield (Random Slot Color)
$29.90
MSI B550M PRO-VDH Micro-ATX Motherboard - AMD AM4, Ryzen 5000/4000/3000 Ready, P
$71.49
GIGABYTE - A620M S2H AM5 DDR5 M-ATX Motherboard
$65.00
HP Z440 WorkStation LGA 2011-3 DDR4 Desktop Motherboard 710324-002
$39.99