In my case the "GO" writing is fading slowly, then hanging for a few seconds (2-3), then I'm getting the annoying red "Error" message, and then I can't re-enter this or any other gym or pokestop for 1-2 minutes since I'm getting the annoying red "Network Error" message.
After a little wait (1-2 minutes) everything is back to normal, without changing or doing nothing much.
Closing the app, turning off the location, restarting the device, and any other action just don't do nothing.
Moreover, if I'll log out and log in again, I'll get the annoying "Authentication Error" message.
I got this bug for 1 gym constantly, and I'm still getting it there - so it feels like hackers are doing something annoying to that gym.
I also already reported this issue to Niantic.
I'm not sure if this is a coincidence, but half an hour after my report the gym next to the problematic gym became empty all of the sudden, without any fight.
Maybe they got my location somehow and missed the gym? Or maybe it's just a pure coincidence, and I missed the fight? Don't know...
Anyway, there's another discussion regarding this issue here at:
https://pokemongo.gamepress.gg/Q-A/gym-bug
