Larian Studios Forums Divinity - Original Sin Divinity Original Sin - Mods Image Heavy - Texture, Model Import, Templates

Try removing the / between Root and Templates. It should probably be
D:/01 - Spiele/SteamGames/SteamApps/common/Divinity - Original Sin/Data/Public/MageFire_63725dce-da32-4474-89e7-722b6f02febc/RootTemplates/MageFire.lsb

Edit:
Yep, that's most likely your problem. I recreated your issue by making my own Root/Templates folder and trying to save there.