since the MonkeyIsland was introduced, generating a new world sometimes takes very long, especially when the world_size is small.
In the logfile you can see alot of "PANIC: missing required prefab [monkeyqueen]! Expected 1, got 0"
I attached a logfile with only client_mods enabled and world_size small.
I made alot of tests and even tried to change the start_mask and fill_mask of the MonkeyIsland-layout via a mod, but for whatever reason this did not help. And although the Hermit-Island is roughly the same size, this island nearly never fails to generate.
With mods that alter the worldgeneration a bit, the MonkeyIsland fails even more often before it finally succeeds, if it ever does.
So maybe you can make the placement code for the MonkeyIsland better somehow?
Btw. Also the terrariumchest setpieces have the same problem, they are also often the reason for "PANIC", but luckily you can disable it in the worldsettings.
Start a new world with world_size small and search the logs for "PANIC: missing required prefab".
Since the generation is random, it does not always happen, but often enough.