Give it a good soak after this brew, use 1/2 L of unscented bleach and top up to the rim. ....plastic can cary nasty bacteria in fine scratches, or around the tap and thread if not scrubbed properly after every brew..sometimes I have been lazy and not cleaned my fermenters immediately after racking,they do develop a fruity smell as you described....be sure to rinse with hot water to wash out the bleach after soaking. Do not place any rubber bits like seals or bungs in bleach for moe than 30min.
Cheers