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When running a T9 Spider lair map and encountering my 3rd of 4 rituals, my client crashed to desktop. I use the standalone client. After rebooting and sitting in queue for a bit, I finished the map, taking my ritual rewards before entering the boss room. There were some mobs in the boss room, but no exit appeared and there is no boss at all.
Luckily, I can prove further that no boss spawned because it was my first Spider Lair, and I did not receive completion for the map.
https://imgur.com/a/YrnOnvw
These are screenshots of the boss room, and my atlas to show non-completion.
Thanks for your report, we're looking into this.