I ran into this issue. "Extension ace_ballistics.dll not found".It would allow me to enter the server but once this message popped up it would disconnect me from the server.
Before launching your game, check to see if you have the .dll files in your ACE folder. Launch the game and attempt to load into a server. If it gives you the error message, close the game then look into your ACE folder and see if the .dll files are still there. If the are not then its your Antivirus that is deleting it.
I currently use Norton 360 for my Anti Virus and noticed that it was deleting my dll files in ACE. I had to manually go into the system settings in Norton and tell it to ignore all files that are in my Arma 3 directory.
For Norton 360 users follow these steps:
settings>Antivirus>Scans and Risks>Scroll down till you see: Items to Exclude from Auto-Protect, SONAR and Download Intelligence Detection. Click on: Configure [ + ] >
Click: Add>Folders>then direct it to your Arma 3 Folder>Click OK
This will stop the Antivirus from deleting the .dll files
