The 2.9.7 release of the plugin was intended to include a fix for the native registration to work with localized (translated) sites. Unfortunately, there was a flaw in the line that fixes this. The flaw causes a breakdown of the native (back-end wp-login.php) registration process.
I have patched this in the main plugin download package. Since I caught this early and the majority of users do not use this backend registration, there aren’t that many users effected.
But if you are one of users effected by this, you have two options. First, as I just mentioned, the download package is fixed so you can re-download the plugin. You really only need to replace /wp-members-core.php (and /admin/user-export.php for one other patch).
Or, you can apply the fix yourself. If editing in WP, go to Plugins > Editor and select WP-Members from the plugin dropdown. Open the file /wp-members-core.php.
Near the end of the file, the second from the last function is wpmem_wp_reg_finalize. Line 920 is the first line of this function and you will see:
$native_reg = ( isset( $_POST['wp-submit'] ) && $_POST['wp-submit'] == esc_attr_e( 'Register' ) ) ? true : false;
The esc_attr_e is the problem. This echos the output and we don’t want that. This should be changed to esc_attr( __( ‘Register’ ) ) like this:
$native_reg = ( isset( $_POST['wp-submit'] ) && $_POST['wp-submit'] == esc_attr( __( 'Register' ) ) ) ? true : false;