Thanks for the answer.
I saw the screenshot, but it only shows that you were able to sign up to the first list. if you now use the Opt-in form to sign up to the second list, it will throw and error message, instead of adding you to the second list, and this only, because you are already existing as a user.
And if you try to sign up for the first list again with the same email address, it will also throw an error message instead of a message that you are already signed up.
And in case you do the same with the regular form via a module, not the opt-in form, it will work in both cases. It would allow you to sign up to the second list, and it would give you a regular response to an already signed up list, instead of throwing error messages.
So it is clear, that the opt-in form is somehow not properly addressing the acyMailing database.
Are you able to reproduce this?
Thanks again..