Reading through here, it looks very similar to an issue I had that wasn’t related to 2018.3, but is instead due to permission levels. Instead of using the “open browser” activity, open the browser by having the robot send the command windows key+r and type in (without quotes) “iexplore”. Then attach the browser that opens and continue as normal.
If it works, it is because the robot is running as a service with elevated access. When it sends a command to open browser using open browser, open application, or start process, it is starting it in elevated privileges. Then, when it tries to automate/manipulate the applicacation it does so at a non-elevated level.
The only fix that we could find was to give very specific elevated access to the user running the robot, or to open programs manually using windows key + r, or navigating through file explorer and running the application executable.