JOEL MADDEN AND NICOLE RICHIE CLASH OVER MARY-KATE

Mary-Kate Olsen showed up at NYC’s Roseland Ballroom to see Joel’s band, Good Charlotte, perform on Aug. 12. “After the concert, Joel invited her back to see the tour bus,” an insider tells Life & Style. “He showed her some baby photos of Harlow. Joel isn’t one to lie or hide things from Nicole, so the next day he told her about it on the phone.”

Bad idea. “Nicole freaked out,” the insider says. “Joel thought she’d respect the fact that he was trying to be honest — after all, he and Mary-Kate are just friends — but Nicole was hurt and angry. He couldn’t deal with it, so he hung up, and she kept texting him after that.”

This isn’t the first time Mary- Kate, 22, has caused problems for Joel, 29, and Nicole, 26. “Joel and MK have a history of flirting,” says the insider, who recalls a recent dinner party: “During the dinner, Mary-Kate was very playful with Joel. It was all fun and games, but with the Olsens nothing is as simple as that. Nicole exploded at Joel and demanded that they leave the party. Afterward, Nicole warned him to stay away from Mary-Kate.”

Unfortunately, he didn’t listen — but Nicole will get over it. “They’re having their usual ups and downs,” says the insider. “I think they’re madly in love and want to get married. But I also think it’s moments like this that will continue to keep their wedding on hold.”

Image WENN