@Jonny_Cook: Apologies for the trouble. Please try re-running the insync-headless add_account command so Insync can get a new auth token for your account. You won’t have to re-download your files or move your Insync folder, Insync will recognize if your account is already added. Hope this helps.
Same here but on Win10 (latest release of Insync and Win) - reauthorizing does not work: after a few minutes it asks me again to reauthorize. For every account I have.
@JohaWeber Are you using a Google Suite account? If so, could you confirm with your administrator if your account has third party apps’ access enabled?