When updating the packages on my Debian I get this error every now and then, and it’s really annoying:

W: GPG error: http://linux.dropbox.com/debian bookworm Release: The following signatures were invalid: BADSIG FC918B335044912E Dropbox Automatic Signing Key <linux@dropbox.com>
E: The repository 'http://linux.dropbox.com/debian bookworm Release' 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.

An internet search about the problem will throw a lot of different solutions at you. Some of them are quite heavy-handed. Some of them just don’t work. Most of them don’t really explain what you are doing when you run the commands you run. I checked a bunch of them and found my own solution.

My understanding of the problem is that packages lists from the Dropbox repository get corrupted for some reason, and this happens way more often with that repo than with any other I am using. When that happens, there may be something left in /var/lib/apt/partial, and it’s better to download the package information for that repository from scratch.

As root (e.g. after running sudo -s or sudo -i), run

find /var/lib/apt/lists -type f -name \*dropbox\* -print | xargs rm

This command will find all files (-type f) whose name contains the string “dropbox” (-name \*dropbox\*) under the directory /var/lib/apt/lists and its subdirectories, and then pass them as arguments to the rm command (xargs rm), hence deleting them.

When that is done, run apt update again and, hopefully, the error will be gone (well, unless the package information gets corrupted once again, that is…).

HTH!