new UserId existing emailaddress => doesn not work and no error message

There is a Problem if the userid is not jet used, but the email address

in
authentificate.php

$new_user_id = wp_insert_user( $exlog_userdata ); // A new user has been created
this Error Object will be returnewd:
WP_Error Object ( [errors] => Array ( [existing_user_email] => Array ( [0] => Diese E-Mail-Adresse wird bereits verwendet! ) ) [error_data] => Array ( ) )

and some lines later:
$user = new WP_User ($new_user_id);
This happens
Notice: Undefined property: WP_Error::$ID in C:\dev\wwwroot\wordpress\wp-includes\class-wp-user.php on line 170

But the User does not recognize whats going on….
With my testuser I had this “Problem” and I needed to debug this until I checked that my original WordPress Adminuser hat the Same emailaddress than my user in the third party DB I want to use for author login…