Permission denied error installing Collabora Online plugin 3.0.1

After updating to to the new Collabora Online plugin 3.0.1 I do get an error in the logs;
file_put_contents(/var/www/nextcloud/core/js/mimetypelist.js): failed to open stream: Permission denied at /var/www/nextcloud/core/Command/Maintenance/Mimetype/UpdateJS.php#127

I’m still able to open documents, but the new functions of Sharing or Inserting a Remote Picture (use pictures directly from Nextcloud) which is new in the CODE 3.4 and available with this new plugin do not work.

Nextcloud 14.0.2