I have this exact problem, except I did not buy the Karts DLC. I did start noticing it shortly after the DLC came out though. I have tried:

1. Uninstalled ARMA 3
2. Deleted both ARMA 3 folders from my documents.
3. Deleted the ARMA 3 folder from C:\Program Files (x86)\Steam\steamapps\common\
4. Deleted ARMA 3 folder from C:\Users\Lang\AppData\Local\Arma 3
5. Deleted everything ARMA 3 from RegEdit
6. Re-downloaded ARMA 3
7. Installed ARMA 3, then ran it.
8. Click on Multiplayer, New, change from LAN to Internet.
9. Picked any mission, including editor (none of them work).
10. I get a blank role selection screen. If I hit ok, it says no players.

Screenshot of my problem.
http://grab.by/xyB4