Almost certainly a permissions issue. There's no simple "do this" answer we can give you out here. What I say in the help always applies - when you setup your new 4.0(5) system, make sure you can add, import and update users, leave them messages and their lamp works ect... then do the restore.
DiRT's ability to synch to the directory has nothing at all to do with the account you are logged in when you run it. It uses Unity's SQLSyncSvr capability which uses the service accounts youc onfigured when you installed Unity. The account you are logged in as only has an effect on message restore (i.e. it runs ExMerge using the local credentials which needs send as/recieve as rights in the mailstore) - but that's not the problem you're having - you're not getting that far.
So start with a clean install of 4.0(5) and make sure you are able to import existing users, create new users and edit them before you pull the trigger on DiRT.
