As you may have noticed by now, I've become a fan and contributor to your efforts. Love the PARTICIPANTS DATABASE plugin, although I'm having a problem.
When starting my test cycle for the form I'm planning, things seemed normal. However, something has caused a derailment. I get:
404 — Fancy meeting you here! ...
This now happens when I press SUBMIT on a correctly completed form.
It also happens when I miss one or more of the required fields on the form. Submission and content checking both worked previously (although I've made form revisions since) and leaving out a required field generated the appropriate red box.
I tried trashing the page and creating a new one. I also tried deactivating and reactivating the Plugin.
The problem occurs under admin login, or anything less. I don't take subscriptions to the site, and inputting the form may be a customer's only use of the wordpress-based portion of my site.
In my setup, I do not send an email to the "subscriber". Their input serves our editor as a source of database information that the editor manipulates thereafter. I use a child theme of RESPONSIVE that contains only simple CSS augmentations...these seemed to work before.
I'll send email to you if you need URLs, etc.... Please just let me know.