When you receive API error 1001: Invalid Authentication this article will help you find the causes and the solutions to it. 

1. Token and/or email address are not valid
2. Invalid domain (API v3)

When you're logged in, click on top right and go to user profile.

Step 2
Do you see tab API Token as below?

If you do not see the tab, it means that the API token is added for you. Contact the owner of the environment and let him proceed step 3.

If you see the tab, go to API Token and verify if the email and token is the same that you are using. If you still getting the error 1001: Invalid Authentication, please contact Nmbrs Support by sending your request to our Dutch ticket form or to our Global ticket form and include the time and date of the error and the API method you are using. 

Step 3
A user account is added on a debtor level for an API connection. This types of user accounts can not see the token that has been added for them. You will have to send them the API token of that user account. 

Go to the debtor which wants to connect to the API. Search in client login section for the mail address that is being used for the API connection. Click on the API token (see example below). Send the API token to the user account. 

If email and the API token are valid but the user still gets the error 1001: Invalid Authentication, check the Nmbrs Status for incidents. If there's no active incident, send your request to our Dutch ticket form or to our Global ticket form and include the time and date of the error and the API method you are using. 

2. Invalid domain (API v3)

Within the API v3, a new field has been added to the authentication header; Domain. The call https://api.nmbrs.nl/soap/v3/DebtorService.asmx?op=Environment_Get can be used to retrieve the valid domain. For more information on the use of API v3, see How to use API v3.