This is my second year facing the same issue with Pro connect, my client has a child that has three last names and the three last names does not stay the way I write, when I e-file it gets rejectec automatically due to the issue with Pro connect's software!! Please fix it and increase the characters since there is a lot of people that has long last names!!
Have you looked at the Social Security Card. The last name (or number of last names) is printed on the 2nd line of the card. My guess is your clients do not really have 3 last names. They likely have two middle names and a last name. I know the parents are proud of their children and get upset when they don't see all of them, but this is sometimes necessary to get the returns processed. https://secure.ssa.gov/poms.nsf/lnx/0110205125#:~:text=Identifying%20first%2C%20middle%2C%20and%20la....
