Recently I began experiencing a error where I could not join a game, with an initial error, then a Steam Authentication error.
Note here that I don't remember the original error, though what I did is easy to follow, and easy to fix.
What I had done to cause this error was in sum:
My Steam and Insurgency files and folder are saved to a second HDD
I reformatted my Windows 10 installation, without keeping any old files
I successfully booted both Steam and I:SS
Joining a server creates the error.
What I did to fix it was reinstall Steam entirely. It was reinstalled directly over the old directory.
This immediately fixed the problem.
I had also tried the flushconfig fix, this did not work.