I just launched a Backdrop CMS site with CiviCRM. When I try to login to the site, I'm getting a "file not writable" error and the site is completely broken for authenticated users. The site is still working fine for unauthenticated users.
My first though was to check permissions on config directory. They appear to be fine. What other file would not be writable and causing the entire site to crash for authenticated users?
What about any of the public, private, and temp directories you're using?
Also check the individual directories where your various cache files might be created (color, css, js, styles).
I would also verify the owner and group are correct for various directories.