Close-up of sign at entrance to Silicon Valley headquarters of outsourcing and freelancing company Upwork, Mountain View, California, May 3, 2019.Activist investor Engine Capital has a roughly 4% stake in freelance marketplace Upwork and is pushing for a shakeup to the company's board.In a letter to Upwork's board on Friday, Engine said the company needs to fix "foundational issues" that are plaguing the platform.Engine stopped short of calling for CEO Hayden Brown's ouster in the letter, which was obtained ...