Toyboy CASPER SMART’s alleged hankypanky with ladyboyz put a definite chill on his romance with JENNIFER LOPEZ; but the major cause behind their blowup was J.Lo repeatedly promising to help her pro dancer squeeze launch an acting career – then never doing one damn thing!

“Jenny From The Block” turned into “Liar, Liar, Pants On Fire,” sneered an insider, who told me: “Over the past two years, Jennifer promised she’d help Casper start his acting career – and even promised to create roles for him in her new movie project, ‘Lila & Eve’ and in ‘The Boy Next Door,’ which she not only stars in but produces.

"Casper told all his friends Jen was going to get him cast in a role in both movies – but all she did was make excuses when it never happened.

"She even promised to create a role for him on her ABC Family series ‘The Fosters’ – which she’s executive producer on.

"When that didn’t happen either, Casper exploded, calling her a manipulative, self-centered, deceitful liar, raging that he now realizes she never had ANY intention of helping him.

"Then he stormed out of J.Lo’s Hidden Hills megamansion – for good!”