A copied world was blocked from loading
Applies to Paper, Vanilla.
What it looks like in your log
World world_copy is a duplicate of another world
What causes it
Two world folders share the same uid.dat, which happens when a world is duplicated by copying the folder.
How to fix it
Delete uid.dat inside the copied world folder and restart.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log