UPDATE 21/1/21: Trading has now been restored to Pokémon Go, following the fix of the issue first identified on Wednesday evening.
Niantic did not elaborate on the issue itself, which likely affected the ability to trade unlimited Lucky Pokémon, but has temporarily raised the game's trading cap to compensate for lost time.
Trainers, we’ve resolved the issue affecting trades and this feature is available again. We’ve increased the max number of trades per day to 150. This increase will last until Jan 24 at 10am PST to make up for lost time trading and earning XL Candy. Thank you for your patience.— Niantic Support (@NianticHelp) January 21, 2022
ORIGINAL STORY 20/1/21: Pokémon Go has disabled the ability for all players to trade creatures with one another, following the discovery of a major bug last night.
Around 11pm UK time yesterday evening, a user on top Pokémon Go reddit The Silph Road reported having traded 100 Pokémon with a friend, with all of them turning into Lucky Pokémon.
(Lucky Pokémon have a higher base stats tier and are easier to power up, and are coveted among hardcore players.)
An hour later, Pokémon Go developer Niantic announced it had pulled the game's trading system worldwide while it "investigated an issue". It's now 12 hours later, and no further updates have been issued since.
Trainers, trading is currently unavailable as we investigate an issue. We apologize for the inconvenience, and will continue to update here. Thank you for your patience.— Niantic Support (@NianticHelp) January 20, 2022
The issue here appears to be that the player traded Pokémon with a Lucky Friend - a temporary status two players can achieve through regular in-game interactions which guarantees the next Pokémon those players trade will be Lucky.
However, rather than the status disappearing after the first trade was completed, the two players retained their Lucky status - meaning further trades also resulted in guaranteed Lucky creatures. More than 100 of them, in fact:
For now, players are waiting for further updates - though there's a few comments from some fans once again surprised by the speed Niantic can act when a bug that is favourable to players is found.