I’m using Nextcloud on unRaid and recently tried version 15. Under the Administration >> Overpage it complains about a missing imagick package.
For improved performance and better compatibility it is highly recommended to install them.
Through a little detective work on the APKINDEX.tar.gz to install it, just use
apk add --no-cache php7-imagick
restart the container and Nextcloud 15 should be happy now.