The update manager gives this error message:
The repository ‘http://apt.insynchq.com/mint serena Release’ does not have a Release file.Data from such a repository can’t be authenticated and is therefore potentially dangerous to use.See apt-secure(8) manpage for repository creation and user configuration details.Failed to fetch http://apt.insynchq.com/mint/dists/serena/non-free/binary-amd64/Packages 403 ForbiddenSome index files failed to download. They have been ignored, or old ones used instead.