Hi Stephanie - I had this issue and found a (not very nice) workaround is to add spaces on both sides of the commas, like this:
Mid("Stephanie" , 1 , 2)
I think problem this due to langue/locale settings where the expression validator gets confused by the commas, and I have raised a bug fix with our product team. In my case I eventually realised that on the BP log in screen I had my language set to
English (Europe), and when I set it back to
English (United Kingdom) the problem was gone.