First off, try navigating to your Arma 3 AppData location - C:\Users\*PC NAME*\AppData\Local\Arma 3\MPMissionsCache
Then delete all Darkside mission files. They will be named "ds_Life**.PBO"
If that does not work, try these tips here - https://support.steampowered.com/kb_article.php?ref=1274-uohk-5653 (Do not try step 3 in "Update your System" section, as if a mistake is made, it can mess up your PC)
If they don't work, can you please post a DxDiag of your system for me?
Do you experience the error on any other games?