I have the same problem, since I updated to Joomla 3.5.1, the emails to user that have registered using the form, they no longer receive the confirmation email.
at the same time, the "thank you message" sends a top line with a message "Wrong adress: {lastname: value}" which is not the adress at all.
I have updated to RS forms 1.51.13, I am not using the Cache from joomla, but nothing seems to work.
I am having 300 users being registered to an event daily and I am desperate since I upgraded 3...