A potential investor in a wood-fired power plant in West Enfield has withdrawn its request to have the state guarantee a $5 million loan amid allegations that the plant’s owner owes hundreds of thousands of dollars in unpaid bills and taxes. The action is a setback for Stored Solar LLC, which has been trying to make its mostly idle plant profitable by locating a shrimp farm and greenhouses on the property, part of an innovative vision of how to revive Maine’s struggling forest-products industry. …In an email Wednesday to the Portland Press Herald, Kimberly Samaha, chief executive officer at Synthesis Venture Fund Partners, an entity allied with Stored Solar, acknowledged the challenges of turning around the biomass plants.