RETALIATION CRASH FIX
Open Mod Manager, go to Mod Management in the top left, click ASI Mod Manager, scrolls down to "Retaliation Glitch Crash Fix" by Mgamerz, then press "Install ASI"
This one is for those poor souls who have the infamous RETALIATION GLITCH (aka the glitch when your game crashes upon checking your DLCs). A big thanks to Mgamerz for ACTUALLY FIXING IT, as well as explaining the nature of the glitch and the reason why the workaround actually works.
What it is and why it happens?
It has to do with the Challenge system added with Retaliation DLC, and happens both on your and on EA Servers' side. It occurs when the DLC initializes and is fetching data from server about achievements (the things on the ticker), and if it hits exception, it endlessly gets stuck checking DLC. If your connection is slow, it'll happen when you click on MP, and then it goes up and crashes right away. So repairing or reinstalling your game, Origin, Windows, or buying a new PC altogether won't help.
Using Mass Effect 3 Mod Manager (if you don't have it yet, you can download it from Mgamer's site ME3Tweaks: https://me3tweaks.com/modmanager/, go to Mod Management - ASI Mod Management - scroll down and find Retaliation Glitch Crash Fix - click on the blue Not Installed button - click on Install ASI Mod. The button should change to "Installed, Up to Date". You can always uninstall the mod should you wish to do so.
This is a work around if you chose not to use Fix1. Before launching your game, go OFFLINE status on Origin. Then launch your game, wait till it loads all DLCs and then go ONLINE status on Origin again. This will make it skip fetching the achievements of others/you, so it effectively eliminates the chance of it hitting the exception and crashing. You should be able to play the game without any problems now. THIS ALSO MIGHT WORK: try to remember what Challenge you had completed before this glitch plagued you, and redo it. Theoretically, this will rewrite your corrupt data and therefore the glitch will go away.