Uploading file to a page creates file with 0 bytes size

I'm using Grav v1.7.43 - Admin v1.10.43. I've just set it up, tried some themes and that all works. Even when I go to theme settings and upload a header image that works, but when I try to add this same image to a page, it uploads it with 0 bytes size. This is with default Home page as well as newly created page. I've tried different file types as well but all get uploaded with 0 bytes. So my upload is working, it just doesn't let me upload to the pages... I don't know what I'm missing, but I tried different setting in pages, adding permissions etc. but nothing seems to work. Any idea what could be wrong?