⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

Hi,
several people seem to have problems when changing the background image. See the last comments in #45083.
Since nobody has added the issue to the list I just did it.
After upgrading my docker image to the nextcloud version 30.0 I am not able to change the background image anymore. If I do so the image will be uploaded but is not applied. Instead I do get an error "conflict between new type (mixed) and old type (string)".

A screenshot showing the error from the menu Desgin is attached

Steps to reproduce

1. Open Management Seeting
2. Select Design
3. Upload a new background image (i tried a 8MB *.jpg)
4. Error message should appear

Expected behavior

The uploaded image should be used as new background image.

Nextcloud Server version

30

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.2

Web server

None

Database engine version

MariaDB

Is this bug present after an update or on a fresh install?

Upgraded to a MAJOR version (ex. 28 to 29)

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response