403 Forbidden client error status response code indicates that the server understood the request but refuses to authorize it.
If you receive this error while trying to update the WP-Members settings, it is usually a result of the settings in a firewall application known as Mod Security.
Many hosts use Mod Security, and often they will apply a generic ruleset. If you are getting a 403 response code when trying to update the WP-Members settings, this is the result of a false positive in one of those rules.
In order to correct the issue, you will need to contact your host and have them apply an exception.