The former 'Idol' and "So You Think You Can Dance" EP filed docs Tuesday, calling out Paula's memory of an alleged 2015 incident in Las Vegas. As we first reported, Paula initially claimed she witnessed Nigel groping her assistant during a 'SYTYCD' shoot ... which Nigel denied when Paula first filed her lawsuit last December.

But Nigel's raising an eyebrow at the fact Paula's story changed 9 months later when she filed an amended lawsuit ... accusing him of also sexually assaulting Paula in Vegas. She claims after the assistant rejected him, Nigel touched Paula's "buttocks, hips, waist and her breasts" and also tried to kiss her on the neck.

In Nigel's new docs, he says, "It stretches credulity that Abdul would supposedly remember witnessing her assistant allegedly being assaulted ... and yet apparently 'forget' until the filing of [amended complaint] that she also allegedly was assaulted at the same time."

Bottom line, he flat-out accuses Paula of manufacturing this new version of her story just to keep her lawsuit alive -- and adds, the "shifting allegations" punch a hole in her overall credibility.

As Nigel sees it, he's the "victim of Abdul's appalling lies."

Of course, Paula's suit also claims there were other alleged incidents with Nigel ... all of which he's denied, and called "false and deeply offensive."