@tommi Did you try the solution described here https://github.com/flatpak/flatpak/issues/3542#issuecomment-610774373 ?
From what I'm reading online, seems to be an issue with flatpak not cleaning up its folders correctly.
@ander_dapo I don’t have major storage problems, but 20GB is a lot… I don’t want and don’t have time to fix potential issues caused by manually deleting stuff without knowing what I am doing, so I will hold off for now
@tommi install Warehouse from flathub and see what app is using how many space
@tragivictoria mmmh but shouldn’t the storage from apps be in ~/.var/app/?
@tommi I'm not familiar with the inner workings of Flatpak. Maybe these links can help?
https://www.linuxuprising.com/2019/02/how-to-remove-unused-flatpak-runtimes.html