Users reporting the same issue on Steam, but with different Workshop item. The only solution is apparently to unsubscribe from ALL Arma 3 Workshop items - rather inconvienient with the amount of modded content used with the game.

I will add that I am also experiencing this error.
A "Veteran" user from the Discord server advised that I fix the issue "by removing one of the files that tracks subscribed compositions". The only file that I could find that displayed subscribed items, including compositions, was a file called " subscribedcontent.list " located in "...\Documents\Arma 3\Saved\steam\meta". It's true that compositions with the ID can be found there, when it is unbinarised. And it's true that the offending (in my case 2) compositions could not be found anymore on the workshop. However, neither removing the specific entries in the file, nor deleting the entire file, fixed the issue.

So let me be clear on the status quo: every time I start the editor, or load back into it, I need to press "close" on the error pop up. I *also* will get an error (for the other composition) every single time I start a mission or join a server, at some unexpected time. Again, I cannot proceed unless I click "close" on the error box. It's technically not game-breaking. But almost designed to be irritating.

I hope the developers find a workaround, since it is inevitable that more compositions will go offline, resulting in more and more error pop-ups.