By default, the following is set in the DirectAdmin configuration:
This ensures that the same IP that submitted the login request is granted login access.
The problem is that some users IPs change so frequently that they are prevented from logging in because of this. In this case, you can disable this check.
Do not do this unless it is absolutely necessary as this will reduce overall security of the server.
You can use the following command to disable this:
/usr/local/directadmin/directadmin set disable_ip_check 1 restart
You should now be able to login without this check interfering.