The number one cause of this error is having "Require Member Account Activation" enabled in EE.
To check your setting and/or disable "Require Member Account Activation" follow these steps:
Log into your EE Control Panel
Navigate to: CP Home › Admin › Members and Groups › Membership Preferences
Change the "Require Member Account Activation" setting to "no activation required"
The reason that account activation cannot be enabled is due to the method EE uses to handle the activation process. EE places a new user in a temporary user group and then changes that user group once an admin has clicked on the Account Activation link for that new user. By doing this it will remove the user from their proper group memberships as controlled by aMember's EE Plugin.
The number two cause of this error is a domain cookie mismatch.
The base URLs of both EE and aMember must match. A common mistake is having the www prefix on one and not the other. If your EE site is linked as http://domain.com/index.php, but your
amember links point to http://www.domain.com/member.php then the
cookies will be different.
To check this setting and/or correct follow these steps:
Log into your EE Control Panel
Navigate to: CP Home › Admin › System Preferences › General Configuration
Make note of the setting for: URL to the root directory of your site
Log into your aMember Control Panel
Navigate to: Utilities › Setup/Configuration › Global
Review setting for: Root URL
The Base URLs of both of these settings must match. IE: http://www.domain.com/... or http://domain.com/...