In the development of the WordPress plugin footnotes, we shipped a broken version that crashed websites for arond four hours starting from 08:37 am. Here is the short version. If you want to dive deeper, check out this forum post
Dear footnotes community
thank you for your continued support. I’m sorry we shipped a broken version of the plugin earlier today and we reverted to a stable version with the latest update 2.5.10. Please update to this release and keep watch the plugin page for further updates.
We’re working tirelessly to bring you an amazing experience and are introducing a test automation and coding standards.
If you want to contribute fixes and enhancements, development is an open process on our GitHub
Best wishes Mark