This seems to be a common problem, but couldn't find any threads here with the same problem..
Origin just doesnt wanna start The Sims 4 at all, and I've been through customer service chat now five times, tried all the troubleshooting tips they had, and I am currently waiting for the "specialist"team to email me, but that will probably take a few weeks.
"ActivationUI.exe" shows up in the task manager for a few seconds when the game is "loading", but then nothing happens, and the applicaiton dissapears from my task manager.. Im starting to fear it might be an registry issue.
Any ideas?

Have you tried a. reinstalling Sims 4, b. reinstalling Origin, and or c. reinstalling Origin and Sims 4?

I have done a manual, complete uninstall of Origin and The Sims 4, I have updated drivers, uninstalled and reinstalled drivers, tried the game in offline mode at Origin, upgraded to Windows 8.1, whitelist the used programs, disable antivirus, run Origin and TS4 as admin, run in windowed mode, run in a different resolution, whitelisting in DEP, changed compability...
Basically, every troubleshooting tip that has been thrown at me. It just seems like my computer isn't destined to play this game.
Thanks for the tip though! Ill try anything that hasnt been mentioned above, haha

I was just reading this post on the EA site a little while ago - http://208.74.205.233/t5/The-Sims-4...out/m-p/3461679
Hope it helps! I read that can help with many Origin issues.

here is the fix that has worked for me. ive had to do it twice...so far. there is no need for you to be deleting, reinstalling, clean booting, or anything that can severely damage your game.
[if your sims 4 game is greyed out and play button unclickable this will work for you.]

1. open origin > my games.
2. on your pc file folder window, navigate to c drive > program files (x86) > origin games > the sims 4 > game > bin
3. drag your ts4 icon from the bin to the desktop.
4. the game cover should now appear normal in origin. click it and choose install. [wait until it says install complete]
5. drag the ts4 icon back into the bin.
6. the game cover will go back to being greyed out and still say install. dont panic!
7. click the ts4 icon in the bin and your game should start. the neighborhood should open your last save before you couldnt play. you wont lose anything.

after you exit the game it should continue to start normally by clicking play in the origin window or the ts4 shortcut on your desktop. however after restarting my pc ive had to repeat these steps in order to play my game again. hope this helps you. i know the feeling of frustration you are experiencing. i felt the same the 2 days i searched for this fix.