Star San is no problem for the Stainless Steel, but it is a problem for the copper brazing material that was used to weld the stack of plates together. It is not a good idea to leave these units in an acid solution like star san for long periods. You may have noticed that the water might have turned a little blue after that soaking.
A short soak or rinse should be OK. Not a long one.
Foam Blowers of Indiana (FBI)
Either it's damaged or it's not. If, when you're chilling, you notice your brew is getting diluted, or when you stop running wort through it, it keeps running; eventually looking like water.....then you have a leak that is allowing water to cross over to the wort side (and vice versa).