The HTTP 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.