Hi there,
I appreciate you reaching out (and for clarifying the steps you’ve already taken, that’s super helpful!)
I am seeing the name and email pre-filled when I visit the page you’ve linked to, which tells me that either there is some custom code pre-filling that field, or some caching still going on. That can also contribute to NONCE errors (as you’ve read).
Interestingly, it is not happening on the individual form page at https://gardeningvolunteers.org/donations/watersaver-landscape-design-school/
(you should see a completed donation and gravity form submission from me)
I just attempted to replicate the problem on my test installation, unsuccessfully.
The fact that it works correctly on the individual donation form, but not on the page you linked to, tells me that some code on that page is resulting in that form being prefilled, which is in turn resulting in the NONCE error.
My first step would be to try embedding a different Gravity form, and see if the problem still exists. Next, I’d try embedding the Donation form in a different page and see if the problem still exists. You’ve got to isolate the problem and make it replicable.
I’m also still leaning toward some kind of server-level caching going on, and creating a new test page and embedding the form in that one would confirm that suspicion.
Either way, I am happy to keep troubleshooting this with you, but at this point I’m unable to replicate it in a test environment, forcing me to believe that the problem is in your site somewhere. Let me know what you find in following my suggested steps above.