Me and my friends tried to figure out, what was causing these problems since the beginning. And we finally found a way to fix those connection problems.

Go to your BL Enhanced Config folder in this directory:
Documents\My Games\Borderlands Game of the Year\WillowGame\Config
OR alternatively check
Steam\steamapps\common\BorderlandsGOTYEnhanced\Engine\Config
(thanks to The Dinkster for pointing that out)

Open up the WillowEngine.ini when you are in the Documents\ directory
OR alternatively
open the BaseEngine.ini when you are in the Steam\ directory
and search (CTRL+F) for bHasVoiceEnabled=True and change it to bHasVoiceEnabled=False




Save and close the file. Go to the properties and mark as read-only (It can work without making the file read-only, but one of us had the problem, that the file got reset everytime he started BL so we just went with that).

The Voicechat in this game uses so many packets that it's causing so much lag.
It's a really small solution but has so much impact on coop gameplay.

Try it for yourself and let me know if it works for you. If not, I'll check for more solutions till the official patch comes out.