On all the sites where I’ve had this issue the following always works
1) deactivate the ACF key
2) do into the db and delete the options _site_transient_update_plugins, _transient_acf_plugin_updates, _transient_timeout_acf_plugin_updates
3) reactivate my acf key
Actually, 1 & 3 have only been required once.
You could also try reinstalling WP as step 2.5, but if you try this I would do it then.
