I am running Linux Mint Debian 6 (debian bookworm) and am running into this issue.
Err:12 http://apt.insync.io/mint faye Release 404 Not Found [IP: 188.8.131.52 80] Reading package lists... Done E: The repository 'http://apt.insync.io/mint faye Release' does not have a Release file. 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.
If I change the distro and release codename on /etc/apt/sources.list.d/insync.list
http://apt.insync.io/debian stable Release
and do an apt upgrade. I am being offered a debian buster labeled package version of insync.
Are there plans to get LMDE Faye added ?