Do you have the latest graphics card drivers?

I do but quite an old card, gts 450.
driver version 314.22.

We need dxdiag and files from this folder for solve your problem. C:\Users\<Name>\AppData\Local\Arma 3 Alpha\
Can you upload somewhere in winrar package please?
When package will be smaller than 2,097k, so you can attach here. Thank you.

http://feedback.arma3.com/how-to-user.html.html

Done, thanks for the quick reply :).

Just a quick note for the future: If steam thinks your game is still running, do CTRL+ALT+DELETE and run Task Manager, and check if arma.exe is running in the Processes tab. If it is, right-click it and click End Process. If it's not there, then steam shouldnt really think its running. Hope that helps.

Thanks for the help but when I check, it's not in the task manager at all!? No processes or tasks :|

Just click on Arma 3 again, it happens to me all the time.

Do you have same issue in other games on Steam?

No, Also the issue seems to have stopped over the last few days.

Outdated. Open a new ticket if new issues arise.