500 error for RSVPs based on markup previously working with no change

I get a 500 error when trying to RSVP to Indieweb events. Still. https://events.indieweb.org/webmention https://artlung.com/rsvp/rsvp20260415pacific https://events.indieweb.org/2026/04/homebrew-website-club-pacific-z7mbQQTEn0Iy gets a plain 500 error with no feedback. The markup I use for RSVPs dates back to March 2025 and is based the markup here. https://indieweb.org/rsvp#:~:text=Here%27s%20a%20minimal%20RSVP%20post%20example%3A It stopped working in late March.

Start by reproducing the 500 error at events.indieweb.org/webmention with the RSVP examples linked in the issue, including the event and RSVP URLs. Trace the RSVP/webmention handling to find why this previously working markup now fails; done means the reported RSVP completes without a 500 error and provides feedback on failure.

Written by the indexing model from the issue text.