On most websites I use a plugin which redirects a user to a Thank You page after they complete a form. This is a trivial function, and not one you might think could bring a website down. I have had six cases over the past week where sites have stopped working due to an upgrade of this plugin. It appears that the authors have substantially rewritten it, and there is probably a bug in there somewhere causing the problem. Can you all check your websites and let me know if you have a message about a critical error has occurred.
That is the symptom. It is easy for me to fix and will take a few moments to do so, but right now I have not been through all of the websites. You can help me to help you.
Normally this type of incident is very rare, and it is clear that not all sites are affected either, so it is probably a combination of things. Around a year ago WordPress made some changes to error monitoring, where a site may have continued to work in the past with some issues, they seem to stop working now.
You may occasionally see a 500 error on your website, these are transient and indicate that the server cannot respond. They will clear themselves. You should never see CRITICAL ERROR. If you see that flag it to me immediately and I will sort it out for you.