Most likely one (or more) of your applications/plugins is not compatible with PHP 8. Check if there are any updates, if there aren't disable all modifications, switch to PHP 8, and then re-enable the modifications 1 by 1 checking which one is causing issues.
Or, you can just wait for version 4.7.2 to come out of beta since it contains a tool that can tell you which modifications won't work with PHP 8.