In the middle the release Repository List has changed.
Now:
stretch→ oldstable
Buster→ stable
bullseye→ testing
sid→ unstable
experimental

So should I restructure the repository list?

I tried to update/Reload Synaptic package Informaiton.

Report:
Could not download all repository indexes

The repository may no longer be available or could not be contacted because of network problems. If available an older version of the failed index will be used. Otherwise the repository will be ignored. Check your network connection and ensure the repository address in the preferences is correct.
The repository 'http://www.deb-multimedia.org squeeze Release' does not have a Release file.Updating from such a repository can't be done securely, and is therefore disabled by default.See apt-secure(8) manpage for repository creation and user configuration details.
Repository 'http://security.debian.org stable/updates InRelease' changed its 'Version' value from '9' to '10'Repository 'http://security.debian.org stable/updates InRelease' changed its 'Codename' value from 'stretch' to 'buster'This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.
Repository 'http://ftp.de.debian.org/debian stable InRelease' changed its 'Version' value from '9.4' to '10.1'Repository 'http://ftp.de.debian.org/debian stable InRelease' changed its 'Codename' value from 'stretch' to 'buster'This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.
Repository 'http://ftp.de.debian.org/debian stable-updates InRelease' changed its 'Codename' value from 'stretch-updates' to 'buster-updates'This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.
Repository 'http://ftp.de.debian.org/debian testing InRelease' changed its 'Codename' value from 'buster' to 'bullseye'

Important points to note: Repository value 9 to 10 / 9.4 to 10.1

Shall experiment and post result here.

The Easy Way: Discard Knoppix 8.2 and accept 8.6?

The Satisfying Way: Re-phrase the Apt repository list with oldstable 9.11.0 as the base.