Yup the latest released version of Geeklog (including the plugins and install) doesn't support PHP 8 yet so those are the reasons for your errors.
We have added some fixes to the repository to fix these issues but the upcoming version of Geeklog is not ready for release yet. (and will still be a while)
PHP 8 Geeklog Issues
PHP 8 Installer Issues
I will add a link to this forum post so it will get tested as well with the trial fix.
One of the Geeklog Core Developers.