With Keep forms alive via a larger continuation memory threshold by jryans 路 Pull Request #80 路 racket/racket-pkg-website 路 GitHub merged and deployed, this should now be fixed. 
The root cause ended up being a memory threshold that was set too low, causing the web server continuations to expire after only 2 minutes, which is why it worked sometimes: you had to submit the form within 2 minutes of loading it. This has been fixed, so you now have much more time to work through the form.