It would seem that Borderlands: Game of the Year Edition is enduring a slightly rocky launch. The enhanced remaster released last week, and a lot of players are running into the same problem when trying to team up with friends.
Indeed, the co-op side of the experience isn't all that smooth right now for some. Many are reporting various connection errors, ranging from not even being able to join a session to being consistently kicked during gameplay. Not ideal when Borderlands is largely built with co-op in mind.
Fortunately, Gearbox says that it's aware of the problem and that it's working on a fix. Let's hope it comes sooner rather than later.