Forge Server Tools · every failure mode · guides
World contains entries for mods that are gone — ERROR
The world was created with mods that are no longer installed, so blocks, items or entities in it have no definition any more.
Either put the removed mods back, or accept the loss: back up the world FIRST, then allow the registry to drop the missing entries. Expect holes where those blocks were. Never do this without a backup copy of the save.
The prompt lists the missing entries and their namespaces, which tells you exactly which mod left.
Accepting the loss looks like a fix because the world then loads. It is not reversible — those blocks and items are gone.
Put the mod back and remove it properly. Back up the world first, every time.
Search your crash report or latest.log for any of these:
Unknown registry entries Missing registry entries registry remapping Forge Mod Loader could not load this save
The free edition covers the six most common startup failures, and this is not one of them — it is in the paid tool, which checks all 24 and names the mod jar from the stack trace. The fix above is the same one it prints, so you do not need it to solve this.