This is what I've tried to solve:
1. Google.com
2. arma3.com
3. Updated all my drivers.
4. Updated Windows to all the latest.
5. Verified my gamefiles trough Steam.
6. Un- and reinstalled the game.
7. Un- and reinstalled Steam.
8. reinstalled the game again.
9. Verified the gamefiles trough Steam again.
10. Downloaded BEClient.dll directly from Battleye's pages.
11. Deleted the profile files in Documents.
12. Formated the SSD disk Arma 3 and Steam is on.
13. Reinstalled Steam again.
14. Reinstalled the game again.
15. Downloaded BEClient.dll directly from Battleye's pages again.
16. Verified the gamefiles throug Steam yet again.
17. Tried devbuild.
18. Went back to normal build.
19. bistudio.com
20. Tried unsubscribing two content subscriptions from Steam workshop.
21. Tried deleting files in "user/appdata/local/arma3/mpmissioncache/"
22. Un- and reinstalled the game after unsubscribing from Steam workshop content.