Under normal circumstances, a Pokemon Go player can receive up to 50 coins a day for having their Pokemon in gyms. Defending Pokemon receive one coin for every ten minutes they successfully defend a gym, although players don't receive the coin bonus until their Pokemon is kicked out by an attacking team.
The issues appear to have started around 2 PM ET today. Numerous threads about the coin issue have popped up on Reddit, including the current second highest post on the Pokemon Go subreddit /r/TheSilphRoad.
While Pokemon Go players have complained about not receiving coins before, most complaints were due to players not noticing a Pokemon returning to them earlier today. However, today's complaints seem to be due to an underlying issue with the game.
Niantic has not responded to an email about the coin issue, but they did acknowledge the issue publicly on Twitter:
Trainers, we’ve noticed a brief interruption in which Defender bonuses are not being awarded. Stay tuned for a fix shortly.— Niantic Support (@NianticHelp) July 17, 2017
In the other side of this story, players have recently discovered that an old exploit can now be used by players to get unlimited coins from gym defenders.
Players have learned that, by changing the time zone on their phones, they can reset Pokemon Go's daily limit on coins. While a player can only earn 50 coins a day from gym defenders, changing the time zones will make the game think that it's a new day and will "reset" the coin limit.
Players can switch between time zones an unlimited amount of times, which means that a player can get unlimited coins...provided they have a steady stream of defenders returning from gyms.
To be clear, this is neither legal nor a new exploit. Some players have been changing their time zone settings to screw with Pokemon Go's sense of time since the game's earliest days. It's also clearly against the game's terms of service and could result in a ban should Niantic ever figure out a way to track players who are using the exploit.
More websites are posting about the exploit thanks to a popular post on Reddit in which a player lobbied Niantic to fix the exploit while giving detailed instructions on how to use it. This likely means that Niantic will have to take action soon, unless they want to see a new form of cheating popularized on the game.