Cant upload anything on child sites but can in superadmin

Hi, I cant actually upload anything as a child site admin. I can upload as a super-admin, and as a super-admin on my child site, but when I am logged in as a childsite admin, I cant upload anything. The bar just freezes at 1% and never completes.

Tried disabling some plugins (but that shouldnt hurt because I can do it as a super-admin) and upload folder is set to 777 permissions

No idea.

P.S.
Interestingly, I can upload an avatar on my profile page (simple local avatar plugin from here)