Thank you page keeps disappearing code left in it's place or blank doc

Hi. The Thank you page for one form is working. For another form, each time I hit submit on the test the Thank you page is either blank or has some odd code. I've wasted 2 days on this. So frustrated. Please help.

I cloned that form and made a test submission, all I can see is the same thank you page on build mode. Your screenshot also shows the same thank you page.

Here's my test:

If your form is embedded, please re-embed it using its iframe embed code, see if this fixes the problem you're experiencing. If the problem persists, let us know the website URL so we can make test submissions in it and see what other solutions we can provide.

Thanks