using you logs I found this topic [bug] hash_hkdf(): Argument #2 ($key) cannot be empty in .../lib/private/Security/Crypto.php:97 - #4 by soilah and corresponding GH [Bug]: hash_hkdf(): Argument #2 ($key) cannot be empty · Issue #34012 · nextcloud/server · GitHub but you post don’t contain enough details to understand if the secret existed before or not.
If you used encryption-server check if this repo helps to recover files: GitHub - nextcloud/encryption-recovery-tools: This project contains tools to recover files that have been encrypted with the Nextcloud End-to-End Encryption or Nextcloud Server-Side Encryption.
In all cases it more recommended to upgrade to a supported version > nc27