I am having this same issue.
https://www.generalcontractor.com/contact/ directs to:
https://www.generalcontractor.com/contact/#sf_form_salesforce_w2l_lead_3
There were no errors on the Thank you page:
https://www.generalcontractor.com/thank-you/
And in fact I even removed the “redirect to custom thank you page” as an option and I am still getting this issue.
Please advise, thank you!