Since a while I get this when doing apt update, I have tried to change repos but it is the same for Siduction extra and fixes anyway. Probably I have missed something here on the forum....?
W: GPG error: http://ftp.uni-stuttgart.de/siduction/extra unstable InRelease: The following signatures were invalid: EXPKEYSIG 15CBD88045C45076 Siduction Repository (The Siduction Repository) <repository@siduction.org>
E: The repository 'http://ftp.uni-stuttgart.de/siduction/extra unstable InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: https://ftp.uni-stuttgart.de/siduction/fixes unstable InRelease: The following signatures were invalid: EXPKEYSIG 15CBD88045C45076 Siduction Repository (The Siduction Repository) <repository@siduction.org>
E: The repository 'https://ftp.uni-stuttgart.de/siduction/fixes unstable InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.