I just ran the upgrade to version 9 via the web interface and now the web interface keeps on loading, nothing is shown. I already tried to restart the raspberry pi bu the problem keeps occurring. What can I do to fix this issue?
You probably was on a version < 0.8 you cannot upgrade to 0.9, you need to install PHP7.0 but it's not easy if you are in old Raspbian.
Download the new image and start from that.