Issue solved! I tried many things incl replacing various .dll files, deleting the arma3 config file, replacing network drivers (somehow accidentally breaking my wireless connection and having to do a system restore), deleting saved maps, changing startup parameters and probably more stuff I can no longer remember.

In the end the solution was simply to close the steam chatbox while connecting to a server. I've had no errors since then. Having the friendslist open is fine and having people message you during connection is fine but if you click on a message or have the chatbox open during connection (including d/ling the map), the game crashed every time without fail. As soon as you are ingame and able to shoot stuff you can reopen chat. Hope this helps others out who have had similar issues.