The first thing to tell us if whether it is a case of

1. Bootloader could not find kernel ( knoppix 'linux' file ), or
2. Bootloader could not find initrd ( knoppix 'minirt.gz' file ), or
3. Kernel could not find knoppix files ( knoppix 'KNOPPIX/knoppix' file.

According to your 'words', it is a case of (1), could not find 'linux' file or possibly the 'linux' file is corrupted. It is easy to confirm that, just do a file comparison of the file with the original. 'cmp', 'diff' or 'md5sum' or 'sha1sum' will also work.

The detail of boot messages will tell you if it is a case of (1), (2) or (3).