I've been through several forum posts related to this issue and so far any solutions found have not worked for me. I'm also on the latest version, as far as I know.
I was able to play online fine the first day I had it, and successfully hosted a LAN server with no issues. When I went to do the same two days later, I get the 'Can't connect to Master' message when I try to log in, and has been that way since.
I am using the free Su-25t and downloaded DCS World from Steam.
Any help would be much appreciated!