Did you try the options in:
File Manager PCManFM > Edit > Preferences > Volume Management > Auto-mount
Unlike all previous versions, 7.6 doesn't automatically mount CDs. I have to type "mount /media/sr0".
Did you try the options in:
File Manager PCManFM > Edit > Preferences > Volume Management > Auto-mount
That doesn't help.
Ok, found the problem. You have to add kernel option:
block.events_dfl_poll_msecs=1000
Looks like a strange command to me but if it works ...
Looks to me like more Linux gibberish. Sure, Linux experts enjoy saying to noobs "you just have to add a kernel option". It certainly demonstrates their superiority and better understanding. But many average users have no idea exactly how you "add a kernel option". It isn't hard to understand why Linux is still marginalized on the desktop.
---
Verifying of md5 checksum and burning a CD at slow speed are important.
At least it's an easy fix, just add it to syslinux.cfg (assuming your knoppix is on a USB stick).
My Knoppix 7.7.0 is a LiveUSB on a Dell Inspiron which has a r/w cd/dvd drive in it.
This implies legacy boot and a remote possibility of systemd messing with things.
I was surprised that cds were not always being recognized, along with other media.
However, I finally noted that, for my set-up anyway, a cd physically in place
in the cd drive reader during boot will be recognized, whereas a cd added after boot
will not be recognized.
So, one work-around for Knoppix 7.7.0 LiveUSB legacy boot users is simply just to
re-boot with the cd in place. I would not be surprised that this problem and work-
around might apply to earlier Knoppix LiveUSBs as well, but I haven't checked this out.

Dell PowerEdge R730XD 28 Core Server 2X Xeon E5-2680 V4 H730 32GB RAM No HDD
$378.93
Dell EMC PowerEdge R440 Xeon silver 4215 2.5GHz 16 GB ram 2x 550W PSU No HDD
$275.00
Dell Poweredge R630 2x Xeon E5-2680 v3 2.5ghz 24-Cores / 64gb / Raid / 2x 1Tb
$374.99
Dell PowerEdge R240 Intel Xeon E-2224 3.50GHz 8GB DDR4 ECC NO HDD 250W
$285.97
Dell Poweredge R810 4x Xeon X7560 2.26ghz 32-Cores | 256GB | 3x Trays | H700
$289.99
Dell Poweredge R630 2x Xeon E5-2680 v3 2.5ghz 24-Core | HBA330 | No RAM or HDD
$149.99
Dell PowerEdge R640 10SFF 2x Gold 6140 2.3GHz 36-Cores 64gb H730p 2x Trays
$599.99
Dell PowerEdge C4130 1U AI GPU Server | 2x E5-2690 v3 | 64gb Ram | 2x 1600W
$524.99
Dell PowerEdge R530 2x E5-2630v4 H330 iDRAC8 Port Card Riser No PSU/RAM/HDD
$99.99
Dell Poweredge R630 2x Xeon E5-2680 v3 2.5ghz 24-Cores | 32gb | HBA330 | 2x 1...
$324.99