php8 is not supported by ownCloud
- It is stated in the Requirements: https://doc.owncloud.com/server/admin_manual/installation/system_requirements.html#officially-supported-environments
- and in the the forum https://central.owncloud.org/search?q=php8
- Search the internet for ‘ubuntu downgrade php’ and find this repo: https://launchpad.net/~ondrej/+archive/ubuntu/php