Authentication issues Windows Server 2016 AD functional level

Dear good afternoon.

I have a problem that I am presenting when installing a Windwos Server 2016 with AD and functional Windows Server 2016, when trying to authenticate with a third-party application Client-Server (developed by the company I work) that uses the domain to authenticate users I can not validate it with a simple user I only achieve validation if I add the user to the administrator group. Agreadesco if someone had and I remember this problem that gives me some information.

Thank you very much.

Sincerely

Andrew.