If it says it's a .NET issue, try reinstalling it. There's a redist folder inside the simulator's installation folder under steam, which includes the redistributable libraries for .NET, directX and something else.
If you want to go all out, try the following.
Revert to the latest official patch (verify local file integrity through Steam).
Clear your cache.
Reinstall the redistributable libraries.
Apply the test patch and hotfix.
Launch the sim and fly a quick mission for a couple of minutes to rebuild the cache.
Restart the sim and fly.
I hope this gets it sorted, cheers
