As a long-time user of our service it is important to know, often when we release certain upgrades we only apply those changes to new inboxes or accounts to prevent upsetting existing setups. One such upgrade was to our Sheets integration when we moved from their v3 API to v4.
The reason you are now getting a 404 error on your Google Sheets Integration is that Google appears to be phasing out the v3 API.
We have reached out to our development team to see if we can attempt updating old integrations, or if we must identify and advise all customers using v3 integrations to recreate their integration.In the meantime, to correct this issue we recommend manually updating your Google Sheets integrations - to do this simply delete the original integration and create a new one.