The form has the FTAN security code, stored in a session. If the session times-out you would get this error.
Also opening any other form simultaneous that uses the FTAN (like backend editor?) will destroy the FTAN, causing this message.
I had my admin open on the page where my form is, but I was not actually working on the form, so there was no other form open. Closing the admin solved the security error, so it seems. I wish I could remove or disable this security crap, as it annoys the hell out of me. It severely impacts the usability of WB.
Not sending emails is a different story.
There can be many reasons (like: using Windows servers with a "From Name" set will fail because of the mail PHP library for Windows)
I just got them, about 30 minutes after sending the form. I guess there were stuck in a spam filter or something. It seems it's not a form module fault after all.