Apparently WHMCS figured that DirectAdmin resellers should not be able to use WHMCS anymore. I just upgraded WHMCS v6 to the latest and it took hours for me to understand why my WHMCS was unable to connect with our DirectAdmin servers.
It looks like you are only able to connect WHMCS to DirectAdmin if you can create Login Keys at User level of an administrator account. So no more reseller accounts allowed to connect, since DirectAdmin uses API commands that only admins have access to.
I can confirm that you are correct, the DirectAdmin module does require admin level credentials for some functions, even basic ones. Unfortunately a workaround for this does not exist as it would require significant changes to the module itself. I would suggest putting in a feature request for this however as I can see how it could be useful for some users, and I haven't seen this requested by any clients previously.
WTF? I'm curious, are people using their own DirectAdmin provisioning module?