Nextcloud SNAP Ubuntu - Read only

Hello together,
I have installed the Nextcloud via snap and now I cannot change any background pictures like “background.jpg” or else. Also in the Theme Area under admin it is not working.

manually change the background.jpg I got the error: mv: cannot move ‘background.jpg’ to ‘background1.jpg’: Read-only file system

Path: /snap/nextcloud/5132/htdocs/core/img

any tips or tricks?