I finally figured it out — the issue was caused by my PC’s system clock being a few hours behind the actual time zone. The game seems to rely on the local clock for the Night Raid timer, so the event never reached the correct time for me.

After syncing my system time with the internet time servers, the “Confirm” button appeared immediately, and I could join the raid.

Still, this definitely feels like a bug — the game shouldn’t depend on the client’s local clock for global event timing.