Yes there is a problem with the this stage of the registration process and the RTB are aware of it. Basically go back to the start and click "No" to the Agent / Management Company.
If this does not work you will have to call them and explain the situation,
This exact thing happened to me over Xmas and I had only 3 days left to register a new tenant, They were closed and so I got billed a late fee of €180. I explained my case to them and they rectified it as they knew the problem was on their side. They waived the €180 fee and billed me the proper fee of €90. Yes it is very frustrating a I tried for almost an hour that night to get past that part.