The following signatures were invalid: BADSIG 871920D1991BC93C Ubuntu Archive Automatic Signing Key (2018) <ftpmaster@ubuntu.com>

Hi, I and other other people around the world are have a GPG error when apt-get update through apt-cacher-ng :

$ cat /etc/apt/apt.conf.d/00aptproxy
Acquire::http::Proxy  "http://172.10.20.30:8080";
Acquire::https::Proxy "http://172.10.20.30:8080";
$ sudo apt-get update
Hit:1 http://fr.archive.ubuntu.com/ubuntu noble InRelease
Hit:2 http://fr.archive.ubuntu.com/ubuntu noble-updates InRelease
Hit:3 http://fr.archive.ubuntu.com/ubuntu noble-security InRelease
Get:4 http://fr.archive.ubuntu.com/ubuntu noble-backports InRelease [126 kB]
Err:4 http://fr.archive.ubuntu.com/ubuntu noble-backports InRelease
  The following signatures were invalid: BADSIG 871920D1991BC93C Ubuntu Archive Automatic Signing Key (2018) <ftpmaster@ubuntu.com>
Fetched 126 kB in 1s (232 kB/s)
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://fr.archive.ubuntu.com/ubuntu noble-backports InRelease: The following signatures were invalid: BADSIG 871920D1991BC93C Ubuntu Archive Automatic Signing Key (2018) <ftpmaster@ubuntu.com>
W: Failed to fetch http://fr.archive.ubuntu.com/ubuntu/dists/noble-backports/InRelease  The following signatures were invalid: BADSIG 871920D1991BC93C Ubuntu Archive Automatic Signing Key (2018) <ftpmaster@ubuntu.com>
W: Some index files failed to download. They have been ignored, or old ones used instead.

I'm using this apt-cacher-ng v3.7.4-1.

Can you please help ?