GEORGE CLOONEY and BRAD PITT are best buddies.  But their friendship might not last forever . . . because they’re having GIRLFRIEND PROBLEMS.

“Us Weekly” says ANGELINA JOLIE hates George’s new girlfriend, former pro rasslin’ minx STACY KEIBLER.  But at the same time, George isn’t a fan of Angelina’s, either.

And things got uncomfortable when the four of them flew on a private jet to a film festival earlier this month.  A source says, quote, “Angelina refused to acknowledge that Stacy was even on the plane.

“Angelina went out of her way to ignore her, from takeoff to landing.  She would not even look Stacy’s way.  She was NOT having it!”

The reason Angelina hates Stacy is because she’s not a, quote, “girly girl.”

And that might be at the root of why George doesn’t like Angelina.  A source says, quote, “He thinks Angelina is boring and not good company at all.  He does not like to spend time with her.”

Source: US Weekly/ Wise Brother Media