Yes, it has happened to us too. We receive the exact same response that the User ID was already taken, after filling out the form. We have been sailing with Princess for 20 years, so there is no new info to advise. I called Princess and they have passed on my complaint.
Meanwhile, if you need to access a booking you can always login under your booking number.
PS We're having problems with the Medallion app too. It just never ends...