Hello Craig,
Make sure that you’re using credentials of current desktop owner. If you want to use domain administrator credentials, you need to log-on to Windows desktop as domain administrator and than authenticate in nxclient’s GUI, if you want to use local administrator credentials, you need to log-on as local administrator. Furthermore, it is not allowed to use blank password account. These constraints won’t be present in NoMachine 5 any more, which uses native Windows behaviour during nxclient’s authentication (if your system allows blank password auth, so will nxclient, the same applies to using credentials of other administrator).
If your problem is different, check these two threads describing similar issues:
https://www.nomachine.com/forums/topic/admin-changes
https://www.nomachine.com/forums/topic/windows7-to-windows7-authentication-issue,
If you still can’t find anything useful, please gather server and client logs using instructions from https://www.nomachine.com/AR07K00677 and send them to forum[at]nomachine[dot]com.