Ok, I figured it out: I moved the contact form into a popup, and apparently that would not work because the form is not present at build time, I guess. So I’ll have to find a way around that…
I think the issue in “Page Not Found” when submitting form must be the same (the code there shows references to a ‘modal’).
If anyone from the netlify team sees this, I’d really like to get an answer on point 3, what is actually supposed to be possible in netlify dev with forms? Because it’s supposed to be supported, but I don’t see what exactly and how.