Sorry to hear about this @simon! There are two things we can do to get started on the troubleshooting process here:
First, post your system information to this thread as described in this knowledgebase article → https://theeventscalendar.com/knowledgebase/sharing-sys-info/
In the meantime, I’d recommend going to your WordPress site’s Facebook Events options and removing the API keys you have already entered in your wp-admin options panel.
Once those are removed, try going through the “setup” process for the plugin again as described here → https://theeventscalendar.com/knowledgebase/configuring-facebook-events/
If you already have a Facebook Developers account, then basically just skip ahead a few steps in that article and proceed from there; you essentially just want to generate some new API Keys for your Facebook Events plugin, and then add them to your plugin options. Once these new keys are added, go to Settings > Permalinks in your admin area, and just click “Save Changes” without actually making any changes. This may seem like an odd thing to do but is a little “trick” within WordPress to get the permalinks across your site to refresh, which might help here.
Let us know what you find from each of these steps!
Thanks,
George