i wanted to play this modpack, but than this stand:

Forge Mod Loader detected that the backup level.dat is being used.
This may happen due to a bug or corruption, continuing can damage your world beyond repair or lose data/progress.
it's recommended to create a world backup before continuing.

what can i do? because when i copy this world the same thing stand there and when i use "Re-Create" than i lose my progress.

Do you have an backup of the world?

not before this bug

Get the level.dat out of the maps folder in the modpack directory and replace it with your file in your save.

It works again, Thank you.

I've deleted the two level.dat files from the maps folder, then replaced it with the level.dat file from my world save file. But that didn't do anything