A regression was fixed in 15.2, which we corrected because the endpoints didn't work on WIndows-based machines. It looks like you are hitting the same error. What is the path set to for your images?
Jacob Overgaard
01/24/2025, 11:57 AM
What do the values in your appsettings.json look like?
The documentation says (https://docs.umbraco.com/umbraco-cms/reference/configuration/contentsettings#login-background-image):
*Login background image
You can specify your own background image for the login screen here. The image will automatically get an overlay to match backoffice colors. This path is relative to the wwwroot/umbraco path. The default location is: wwwroot/umbraco/login/login.jpg.
Login logo image
You can specify your own image for the small logo in the top left corner of the login screen. This path is relative to the wwwroot/umbraco path. The default location is: wwwroot/umbraco/login/logo_light.svg.*
So if I read this, I'd say you need to place your files in wwwroot/umbraco/backoffice for it to work.