docs not downloading on email to kindle?

Hello, I use a kindle fire HD II (yes I know it's like 15 years old but it's brilliant) and suddenly this week the email to kindle feature has stopped working.

I've been emailing it work or research docs (usually PDF, sometimes epub) for years because of the seamless sync, but as of 3-4 days ago, all of them show up errored on the device with a "failed to download" and (i) error icon.

To be clear, I email the PDF to kindle, verify the send via amazon inbox, the file shows up in the kindle carousel but it hasn't downloaded. I've tried everything I can think of, restarting, it has enough clear space, etc. Weirdly I can copy the files manually with 0 issue, it's just the email sync that's stopped working.

Anyone else have this problem or know how i can fix it?