Something must have changed, things don't suddenly happen.
Have you installed any add-ons recently? Made any changes to the server? Use Cloudflare?
Try disabling all add-ons and using a default unedited style and then try posting to see if it works. If it does, then enable them one-by-one to find the culprit.
Also check the browser console (right click and Inspect if using Chrome) when you get this issue to see if there are any errors (I notice that there are 3 warnings when logged out, so there could be errors when logged in).