PopoPopo

Welcome back, I've solved my issue. As I suspected, my World Edit was messed up, thus I had to clean it.
To help anyone who might be in the same situation as me, here's what I did:
1. uninstall the game + manually delete your \VintageStory and \VintageStoryData in your Roaming folder. Don't forget to keep your save somewhere on your pc!
2. download back your game, and put your save back in your \VintageStoryData\Saves folder.
3. run your save in repair mode.
4. try the command /gamemode survival. You must be close to the floor if you don't want to die from fall damage.
5. if it works, good news! Now write the command /op xxx, where xxx is a name (eg. /op Popo). I can't explain why, but it seems that it has cleaned my World Edit. -> check https://wiki.vintagestory.at/index.php?title=List_of_server_commands#/op for info.
6. now, the command /gamemode creative should works fine.
If it's not working for you, I'm so sorry.
And for every one, let this be a reminder of doing backups and saving your works from time to time.