Unable to remove or change background image from sign-in site

Question

I have configured my sign-in site for my tenant some time ago. I wanted to change the bacground image recently and used the company branding option in Entra, user experiences> company branding. I removed the existing image and uploaded a new one in complinace with the demands (image size, file type, max file size)

• I saved the new image but nothing happend. I waited 24 h to see if the issue was related to updates in the backend but the result was the same.
• I then tried to delete the image and save without an image at all, same result.
• I then changed the layout from full screen to partial-size screen, these changes took effect asap but the old image still remains as background image.

I have opende the sign-in page in different browsers, inPrivate/ Incognito mode, I also cleared all browser cache and tried in- and outside the company network.