Office 365 can’t modify files directly on dropbox

I try to get my office 365 installation on my Android 10 (Huawei) Smartphone to modify excel files that are stored on dropbox.

But the files are allways read-only. Even if I create a new one on the android smartphone, they are read only after saving.

I can’t find something about this via google, thats why I think it is some minorr issue.