Guest users prompted to enter E-mail and password

Hi, 

since yesterday our participants aren't able to visit our Webinars the way they were used to. Our Webinars are set for guest users. We checked the settings and we didn't make any changes but now the participants get prompted to enter their Mail and a Password after registering and clicking on the link provided in the registration E-Mail. Did Adobe change anything the way participants are handled? If not, what can we do to get back to the status we and the participants are used to?