I'm having the same issue, it's like we hit an "invisible" limit for the Arma 3 Launcher. Can still load all the correct mods, and start the game, and THEN join on the server. But it's still wierd when it gives Signature mismatch on the list. (Problem happens when adding 1 more key)

Actually not. Its tied to specific keys. I had tested that adding specific key triggers that problem.

Hmm, we did some more testing on our server, and did some deep digging on the internet, and it seems there's a limit to 25 keys on the server.
At least that's what we've got. Add 1 more key, and everything "breaks", removing any of the other keys will "fix" it.

im at maybe 15 keys. but adding any of the keys in that zip always breaks that system.

Will there be any response from the Devs ?