Had no issues about a week ago when last tried to log on. Now when i try i get a steam popup saying game infor and the IP address for the server and other info that i never got before, would usually just launch the game. but when trying to join now through that popup, i get another popup saying "Failure - Unable to connect to server, app id specified by server is invalid" 

What game?

I've got the same problem, on Ark. I can't launch the game via the website, nor can I find it on the server list. It simply says invalid ip. 

To anyone with the same problem I just fixed it, or I found someone who fixed it. This is what his fix was: "A friend of mine found the solution. You need to start ur server then go to "View" in the top bar in Steam, go to game servers then to History, right-click on ur server, and add it to favorites (just so it stays there). then you can join over this overlay after starting the game" This isn't exactly what I did, when I selected ark I added the server IP, and then I could launch the game, I found the server ip in the fail message. 

Yeah, getting this same issue this morning when I was able to connect two days ago.

Yep. Steam changed some things involving their SteamConnect plugin, and it no longer functions the same way for several games it seems.

For now, until it all gets sorted you'll want to add your server to your Steam Server Favorites manually.