The same happen when i try to join my friend game/server.
I have deleted media.zip and then checking the integrity of the game and after donwload the file with no luck and even a complete new install ,some maps do works and letme join while others dont.
Unless some distributions of the game are some how older the others ppl game and creating this problem i dont have a clue.
I've only played multiplayer with friends but between the four of us we have all been getting this .STG error on every attempt to play together. Trying to get everyone together into a game, even with no mods being installed, eats up a lot of time and leads to a lot of frustration.
We've tried all sorts of tricks. Verified game file integrity through Steam. Host launching then players connecting directly through Steam. Having different people host (best success here when the host is someone with a poor connection.) Joining the game while leaving the Steam overlay active. We've tried all starting out in the game lobby at the same time, we've tried joining after the game has started.
Typically with four of us only the host and one other player makes it in right from the start, the other two will have to try joining many times before it works. With just three players total it's very common for only the host to make it in right away.
Even with just two people, the host and one other player, it can take multiple tries to connect. I'm really surprised that this issue doesn't seem to be getting more attention, it's been terrible! We want to play this together and we simply are not able to.
What makes it all worse that there is no single word from anyone of the game team like announcing yes we know and we are working on that. It is as they went for a all time party after releasing the game. I bought it with some hopes it's gonna get better support than it had uner Oovee... so far I am mainly disappointed.