Jennifer Lopez has had nothing but nice things to say about Ben Affleck’s ex-wife Jennifer Garner, calling the actress, 50, an “amazing co-parent.” But when J. Lo, 53, and Ben, 50, went to invite Jen and her boyfriend, John Miller, to Thanksgiving at their house, they didn’t get the reaction they were hoping for. “While Jen has accepted J. Lo and Ben’s marriage, she feels it would be awkward to spend the holidays together,” reveals a source. “She wants to keep it separate.” It’s not the first time J. Lo has felt slighted by Jen. The Alias alum, who shares daughters Violet, 16, and Seraphina, 13, and son Samuel, 10, with Ben, is said to have turned down an invite to the couple’s wedding, too. “All J. Lo’s efforts to…