I have previously added 5 new users in the Account Administration area without problem.
Today, I have tried to add another address which is also registered and verified as an Adobe ID, but whenever I submit it, I am getting the message 'Sorry! An error has occurred. Please try again'.
Is the new user's Adobe ID already verified to use with DPS? If so, that could be the problem. Use a valid email address that hasn't been connected yet with DPS.