We are receiving this error message almost constantly when trying to submit a form from a number of our landing pages. Campaign is published, landing pages are complete. Refreshing, exiting the page and returning, re-entering details, different computers, none of this seems to help. Why is this happening? This is beyond frustrating!
I can totally get how frustrating this must be for you. Are you using the Landing Page builder, the web form submitted or the old landing page builder?
Are you using these as hosted (ie providing a URL) or embedding them in your website?
All the best