Since upgrading to Ubuntu 17.10, I’ve been getting the following error when I try to run an apt update:
E: Failed to fetch http://apt.insynchq.com/ubuntu/dists/artful/InRelease 403 Forbidden E: Some index files failed to download. They have been ignored, or old ones used instead.
Do I need to update my configuration?
We’re still currently working on the 17.10 build. Please stay tuned for updates.
Have you tried the 17.04 version if it works for you? 
The issue with using the 17.04 version is that it automatically adds a repository using the current release name (artful) that doesn’t yet exist. When this happens, every time the user runs apt-get update, error messages occur. The solution is to remove the repository from the apt-get sources, but if you do that you can’t install the insync-nautilus package.
Insync might want to consider doing what Crossover Linux and Google Chrome does, which is to forget about repositories, and supply ONE package for debian-based systems. Crossover doesn’t even have a repository — if you want updates, you got to their site and download a new version. Google does have one common repository because their package works on all debian-derived systems. This makes my life a lot easier, because I run Debian and Ubuntu and the same package works — I don’t have to keep multiple versions around.
If you don’t want to do this for various technical reasons, you probably need a repository up and running for new versions of Ubuntu as soon as Beta 2 hits. That’s when people start to make the switch. Definitely a repository needs to be set up by the release date, which was 10/19.