Guys, it may be that the newest DLC messes-up Mad Moxxi's Underdome DLC.  
 
The game hit this stupid bug, which basically made me not get my final Achievement... that I'd be probably be getting any minute now if it weren't for this happening.  
 
Ah, well.  I made a video if anyone wants to check-it-out and just have a heads-up.  If this happens to anyone else, maybe we should let Gearbox know about it?   
 

Don't use cheap cheats?
Split screen lol

Stop wasting time.  Crawmerax must die!

did you try before and not have this happen? seems weird that one would cause the other.
 
i have my own share of moxxi problems, and it's been going on before knoxx was even announced :(

actually maybe you could help me out.
 
basically i took the quest for mad moxxi, prove yourself, and i completed it online with a friend. but when i go back to the bounty board after turning it in, it's red like there's no more missions
 
but the 20 round tournaments opened up in that little back area. is there just not a mission for those tournaments or am i totally fucked?
 
i've done all 3 of the original 5 round arenas again, without the mission, on my own in single player, hoping that would fix the problem, but nothing happens.

" actually maybe you could help me out.  basically i took the quest for mad moxxi, prove yourself, and i completed it online with a friend. but when i go back to the bounty board after turning it in, it's red like there's no more missions  but the 20 round tournaments opened up in that little back area. is there just not a mission for those tournaments or an i totally fucked?  i've done all 3 of the original 5 round arenas again, without the mission, on my own in single player, hoping that would fix the problem, but nothing happens. "

" @onimonkii said:

" actually maybe you could help me out.  basically i took the quest for mad moxxi, prove yourself, and i completed it online with a friend. but when i go back to the bounty board after turning it in, it's red like there's no more missions  but the 20 round tournaments opened up in that little back area. is there just not a mission for those tournaments or an i totally fucked?  i've done all 3 of the original 5 round arenas again, without the mission, on my own in single player, hoping that would fix the problem, but nothing happens. "

Damn, dude... that sounds like a way worse bug than mine.  At least with me, I can restart it over again and I only lost like 1.5 - 2 hours of work.    But anyway, all is not completely lost for you, I believe.  What you can do is start a new character and get them to the first Fast Travel Station, then go to Moxxi's Underdome and then exit/save.  Then make a new split-screen game with that new character hosting and then join it on Player 2 with your regular character.   The new character can do all the Starter Tournaments again and should open the back up, then help you take-on the Big Tournaments too.  Hope that works!  "

just as an update, i joined a random dude in a game online who was doing moxxi, he had done the first 2 arenas but was in the gully when i joined, 5 rounds
 
when we finished, all 3 arenas checked off for me, and i could turn in the quest. it gave me another skill point, but still didn't unlock the 20 round quest
 
now that i have the extra skill point? kinda don't mind. haha