This usually means the mod has been updated, so doing a repair in the Arma 3 Launcher will most likely fix the issue. It can also happen that mod files, or their signatures, got corrupted. Repairing the mod in question will also fix that issue
If it’s an Arma 3 core pbo you will need to verify the game files in Steam instead. See the Steam Support page: https://help.steampowered.com/en/faqs/view/0C48-FCBD-DA71-93EB
Also keep an eye on the General Chat and Help channels on Discord, because it also might be that the server hasn’t been updated yet. In that case wait till J9 has solved the issue, or contact them so they can resolve the issue.