It seems to work fine for me on an embedded form: it shows every time until i check the box. Then its gone on all other forms as well where i use the same checkbox.
Right. This is actually a drawback of checkboxes w/ProgPro in other cases (because they will never go away, despite being, let's say, "positively omitted") but for your case it is convenient.
Dory´s and Dan´s suggestiong (in this thread) with the "OptIn Date" field condition did not work for me (assuming because its a non-marketo page). Can you confirm this?
Without PreFill, Opt-In Date can't be used to trigger visibility rules.
But I agree with Dory. It's always better to open a new thread, because since you aren't the OP you can't ever mark anything Correct here. It's better to have new, closed threads with up-to-date info than old open threads that never go away (and often contain obsolete and/or incorrect information).