You need to fix the Permission of the ubooquity appdata folder. You only have read Permissions, after giving the folder write permissions you can copy the themes.
I have the same Problem with my nextcloud. It looks like the Document Server updated app to not work properly.
I solved this by disabled the "Document Server" with the terminal.
Open the nexcloud terminal window and type this command
Lists all your current isntalled apps
occ app:list
then I searched for the Document Server and used this command to disable it.
occ app:disable documentserver_community
Now I can login to nextcloud and see the user interface again.
Go to this folder and edit the config.php file with notepad++ and change the "maintenance" from true to false. restart nextcloud docker
\\SERVER\appdata\nextcloud\www\nextcloud\config\config.php
'maintenance' => false,