Pokemon-go – Unable to place pokémon into a gym

Were you at a location where GPS accuracy may have been "suboptimal"? What I experienced often in those cases, is that your location bounces around the area when your GPS updates and decides that you are now in a different location. If this temporary new "wrong" location is too far away from the gym, it may cancel ongoing fights.

In this case, you may sometimes still see the "Gym too far away" message in the gym main menu for a short moment.

Unfortunately, no, the prestige was never actually at 60k, despite being displayed that way. Prestige never actually is higher than 50k, regardless of how it appears. See this post:

The gym is not actually at 52k prestige - this is simply a bug. The gym is actually at 50k prestige, but due to the way the game handles adding a Pokemon (adding +2,000 Prestige each time) when you add the last defender possible to a Level 10 (maxed out) gym, it makes it go from 50,000/50,000 to - artificially - 52,000/50,000. It really is at 50,000 though. So if you were to attack it, according to recent findings (somebody tested this), it should actually drop down from 52,000 to below 48,000, resulting in somebody getting kicked out (it's just that the total Prestige drop displayed to you may not always match up, essentially because of the bug giving you false numbers to begin with).