The following mods may need validating/updating
This is the error I get everytime I download mods for a server and then hit play. I open up arma in steam and its says all mods up to date. Little confused here... I just want to join a server. I install the mods, load them and hit play.
In order for Steam mods to download you cannot launch ArmA 3 from steam, I personally have allow downloads during gameplay ticked since I have a high speed network connection saving me closing the launcher etc but this may be why you are seeing the issue, also be sure to check the mods tab in A3Launcher to make sure nothing is busy doing anything.
P.S: "all mods up to date" in the ArmA 3 vanilla launcher does not have anything to do A3Launcher's mods.