Looks like the Carbon team messed up, they tagged the wrong code as an update to the 1.x serie briannesbitt/Carbon1708
HilbertGilbertson sounds like a valid fix while we wait for Carbon to fix their mess 💩
UPDATE: Carbon deleted the faulty tag. No more workaround should be needed if you install or upgrade Flarum now.
If you had applied the workaround and want to upgrade to the latest Carbon once they re-tag it, you'll have to remove the carbon line from composer.json. You will probably also need to remove this line when upgrading to beta 9 when it will be released.
UPDATE 2: Carbon tagged version 1.37.1 . If your Flarum is in a state of error, run composer update nesbot/carbon to get the fix.