It’s worth to note that the latest update at this point in time was no “normal” update like the ones coming along from time to time, but the major upgrade from Android 7 to Android 9 as the topic title suggests.
So …
- backup all your important data or make sure it is successfully synced to Google or whatever internet cloud service you might use.
- Do a so-called factory reset, with Android this will delete all user data and Apps you installed and their data, the OS will be left as it is … https://support.fairphone.com/hc/en-us/articles/115001610926-How-to-make-a-Factory-reset-on-Fairphone-2-
- If this doesn’t help, reinstall the OS … https://support.fairphone.com/hc/en-us/articles/360018801738-Fairphone-2-A9-Manually-install-Fairphone-OS