Game works fine in Editor, crashes in a build.

So my game works 100% fine in the Editor. But when I build it, and run it, it crashes (at a certain point, not directly after opening the game). It crashes when I press my Login button, which runs the following code:

         try
         {
             ConnectionInfo info = new ConnectionInfo("127.0.0.1", 8886);
             TCPConnection connection = TCPConnection.GetConnection(info);
 
             string response = connection.SendReceiveObject<string, string>("LoginInfo", "LoginRecieve", 1000, playerUsername + ":" + playerPassword);
 
             if (response == "Success")
             {
                 //Username and Password were correct. Stop showing 'Loading...' and show the LoggedIn UI. And set the text to display the username.
                 UserAccountManager.instance.LogIn(playerUsername, playerPassword);
                 ResetAllUIElements();
                 loadingParent.gameObject.SetActive(false);
             }
             else
             {
                 //Something went wrong logging in. Stop showing 'Loading...' and go back to LoginUI
                 loadingParent.gameObject.SetActive(false);
                 loginParent.gameObject.SetActive(true);
                 if (response == "UserNotFound")
                 {
                     //The Username was wrong so display relevent error message
                     Login_ErrorText.text = "Error: Username not Found";
                 }
                 else
                 {
                     if (response == "InvalidPassword")
                     {
                         //The Password was wrong so display relevent error message
                         Login_ErrorText.text = "Error: Password Incorrect";
                     }
                     else
                     {
                         //There was another error. This error message should never appear, but is here just in case.
                         Login_ErrorText.text = "Error: Unknown Error. Please try again later.";
                     }
                 }
             }
         }
         catch(Exception e)
         {
             Login_ErrorText.text = e.Message;
         }

The game crashes with this error: I do use NetworkCommsDotNet, and it may be a problem. This is my log: https://cdn.discordapp.com/attachments/284162864632299522/372726341742755840/error.log

Thanks in advance!