Have received this error to connect to Halo 5 servers for 5 days now. It isn’t on my end. Any ideas?
Try restarting(from the options) the Xbox, fixed it for me 
As far as I can tell, if it’s not on your end (hard reset/connection reset don’t work) then it’s on the server end and it must be from the high holiday traffic and their servers not being able to handle it.
I’ve had the same issue with a second to none fiber internet connection.
It’s on/off for me as well! It must be connected to holiday traffic somehow, as these problems rarely happened pre-December
I’m not playing this game until they fix the damn server issues its RIDICULOUS. Just went 2 and 7 when I was lagging wouldnt let me respawn yet I kept dying while I was outside of the map at the rig didnt wanna quit for fear of ban but my God my KD goes down for nothing.
This has been troublesome for me at times, too, unfortunately.