APIM, Can not make NTLM authentication work

Document ID : KB000125090
Last Modified Date : 22/01/2019
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Question:
We have setup an Identity provider, with the provider type McirosoftActiveDirectory. it passes the test. In the Enable NTLM step, we setup the NTLM settings. We used an computer account as service account. When we test the connection it does seem to pass as successful. When we test the identity provider by searching it for an user. It does return the right user. So the identity provider seems to work as designed. When we use the Require "NTLM Authentication Credentials" assertion and select our identity provider, and enable the service.

We see a proper login prompt in the web browser to input the user credentials. But it does nog seem to authenticate any user account we put in.  We tried different ways of formatting, still no success. And yes we also fill in the domain name. What can be going wrong?
Environment:
9.3 and 9.4
Answer:
Netbios name of the domain was too long; the actual name contained more than 15 chars.