Let’s say for example you have Nextcloud configured to make server-side encryption to external folders (as intended: for example dropbox). You made backups of the encryption key (single master encryption key).
Then for some reason, your Nextcloud server is compromised and unfortunately, you did not make a backup of the server: e.g. a major problem with the server hosting platform, a robber stole your Raspberry Pi or your cat decided it does not belong on the table.
Would you be able to retrieve the data with the keys but without Nextcloud?