Error with Theming App

Hello, I´m new to Nextcloud and currently working on Nextcloudpi. I tried to change the login image (=background) but if I upload the file I get the error “Missing a temporary folder”. I already tried to convert the image and even put it in a zip file, but nothing worked. I searched for a solution like 30 Minutes now and it seems like no one else has that problem. Can you plese help me?