Been like this all weekend? What the hell is going on? Both on Arena and WZ I cannot get in to a match at all.
Me to i have the same problem you are not alone. We hope 343 fix this soon. I leave here my issue to compare:
--------Hi, im from Guadalajara, Mexico and i have one big problem.I can´t join Matchmaking because when i choose any type of game it apears “Imposible to join, time out” in spanish “Imposible de unirse, se agoto el tiempo de espera”, i have Totalplay but in the past i never had this problem with this Company. I rebot all my xbox, my Halo 5 game, my account and my modem and nothing.
PLEASE! If you can help me to repair this issue i´ll be very happy to continue playing Halo 5.Thanks.------------
It’s about to be 3 days now and STILL NO FIX.
I cant play since 2 weeks.

It´s very frustrating because you can´t report it with 343 Telephone Numbers.
Helios68, you have “Avoid Me” rep on Xbox Live. This will affect your matchmaking, as you won’t be matched with people who have good rep. You will only be matched with other people who have “Avoid Me” rep (there probably not enough red reppers online at any one given moment for you to successfully match, at least not often), and this applies to all games, not just Halo 5.