Fans who were hoping to catch ’em all at the Pokémon Go Fest today are experiencing some technical issues. The real-world event is developer Niantic Inc.’s way of throwing a one-year birthday party for its hit mobile game Pokémon Go.

Around 20,000 fans are expected to attend the festival today in Chicago, Ill.’s Grant Park (as first reported by news site Game Informer). The event, however, had a rocky start when many were unable to get on the servers for the location-based monster-catching game. Fans, many of whom still play the game every day, showed their frustration with the outage and the developer by booing Niantic CEO John Hanke when he took the stage to address the crowd.

The festival comes after Niantic rolled out new features, such as an updated gym-battle system, as well as highly anticipated legendary Pokémon, which players will be able to catch through group raid battles.