DropboxResponseError: Response failed with a 400 code

That looks like a valid long-lived access token. Can you double check that this is the exact string being used? Can you share the rest of the relevant code where this is used for reference?

By the way, note that access tokens enable access to Dropbox accounts, so for the sake of security, you should never share access tokens like this. I've redacted it from your post, but since this access token has been shared here, you should disable this access token. You can do so by revoking access to the app entirely, if the access token is for your account, here:

https://www.dropbox.com/account/connected_apps

Or, you can disable just this access token using the API:

https://www.dropbox.com/developers/documentation/http/documentation#auth-token-revoke

Note that doing so will cause any further API calls made using this access token to fail. This cannot be undone, and you would need to get a new access token to replace this one.