Looks like I finally found an error similar to mine. I am also trying to get two working mailchimp forms on my wordpress website. I am doing this without plugin’s. If you take a look at me site you will see that there are two signup forms. The first one works exactly the way i want too (it shows a message after you have completed it that you need to check your e-mail) the second one however opens another tab with the standard mailchimp sign up thank you page.
The code is exactly the same for both forms. On another page of my website I also use the second form and it’s code and there it does work the way I want to.
I’m interested to hear your opinions or possible solutions.