Niantic has updated the official Known Issues support page for Pokémon GO. Read on below to learn more:
Pokémon GO Freezes after Remote Raiding
Issue Description: After updating to app version 0.311, Pokémon GO freezes when attempting to join a subsequent Remote Raid from an invitation after participating in an initial Remote Raid from an invitation.
Issue Status: Investigating.
Confirmed Workaround: Restart the app to join the Remote Raid.
Source: Official Pokémon GO support page
Source:Pokémon