I'm not sure what the error message means, without seeing a copy of your database - but it may not be necessary
You don't need to add an AD user to ePO in order to run the AD tasks - instead, all you need to do is specify he AD user to use when you set up the registered server. ePO will then use these credentials when the task runs. Give that a try - hopefully it will solve the problem
We have found that we are getting the "unexpected error occurred" message not only in adding users, but in setting up tasks and policys etc.
In that case, please open a case with support - we'll need to see your database at the very least, I'd imagine.