The plumber is right. You obviously have a broken drain pipe leaking septic waste water into your sump pump drainage system.
I'm afraid that you're going to have to cut or break up the concrete basement floor to find and repair it.
If it's a "recent purchase" and an obvious long-standing problem, it should have been disclosed to you by the seller (and their agent, if there was one). You may have recourse against them, if you can find a plumber that they called to check into this (known, but hidden, undisclosed problem).