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Server owners typically encounter this error when running modpacks on their server and switch between various packs. When switching between different versions of the modpack or switching packs, you must also reset the world. If you don’t then you will encounter an error in the console similar to “this world has been saved with this mod but is not longer installed. Things may not work well.”. As the statement would imply, running a world with modded items installed on it but no longer with the same mods installed on the server will evidently break the server as it tries to load a world up with items it can’t recognize.
These items are referred to as ghost entities since entities remain even after a mod is remove. If these block remain in your world, your server may have difficulties starting up. Most of the times you can bypass this halt on your servers start by doing the /fml confirm command in console. What is basically does is bypass the halt by forcing you to acknowledge the issue before loading your world. By doing this, it acts as a safeguard against possible corruption. This option is not always a sure fire fix to the issue. Sometimes your only option is to reset the world, removing the ghost blocks entirely and allowing the world to generate anew without them.
To definitively fix this error you must reset the world or revert back to the previous version of the modpack or set of mods that worked with the world.