When I try to create a world using the "Better X" preset, the world crashes. (when I use the "deafult" preset it works normally.)
crash-2022-07-21_01.19.53-server.txt

is there any way I can make the default world type "default" again, instead of "BetterX"? (I saw that in the "bclib" settings there is the option "forceBetterXPreset [default: true]"... I changed it to false but it remained the same.)

That setting is currently only for servers. Without the BetterX presets our mods will not work properly (some features will be missing)

Judging from the logs, the crash comes from wythers:desert_river-Biome and byg:firecracker_shrubland. Somehow features for both are added in an inconsistent order, which will crash the game.

Ok, I think I know why this only happens with the BetterX world type, and it is a feature not a bug. :)

From what I saw in my testing wwoo and bug are not compatible.

When you choose vanilla, you will simple not generate any Biomes from wwoo. BYG will take over the Overworld. That means the wythers biomes will never get initialised which in turn will not result in the inconsistent feature placement and the crash does not happen.

With BetterX you will still not get any Overworld-Biomes from wwoo (as we do not change the overworked behaviour). But we sort of run a sanity check, which surfaces that incompatibility.

can you explain how the "forceBetterXPreset [default: true]" option works?

That option should be a part of the server.json config.

On a server, this will always enforce the BetterX-type (no matter what is configured in server.properties). If it is disabled, the world-type setting from the server.properties is honored.