I just need a nudge in the right direction!
I have a list with comment question where if the answer is No, I want to make the comment mandatory, else comment isn't mandatory.

The question is like:
Do you want to continue? Yes | No
And I figured I could use a question validation equation to force comment mandatory if the answer is No, like this:

Question validation equation is ((!is_empty(self.sq_comment) AND Q1=="A2")) OR (Q1=="A1")

But if I select "Yes" (Q1=="A1") I am still told I cannot submit the question.

What might I be doing wrong?

Why not add a following long-text question shown via relevance?

Usually, I would -- but in this case, the list with comment question is used in a quota to early terminate the survey, so a following question won't be shown if the response is No, because the survey will terminate before the text question is shown.

On the same page?

Besides the suggestion of Tpartner, you could put the screenout a little later via equation question, to avoid your issue.

Right, so basically I could use equation question to set the value of a hidden Yes/No (eg: code=QuitNow) type question to Yes/No based on the response to Q1, and use QuitNow with quota, correct?

No, depending on your version of Limesurvey, you could use the equation directly for your zero quota, no need to set another question type anymore, afaik.

I don't see the need for an equation question. I have tested my solution and the value in the screen-out radio and following text question are saved after a screen-out on the first page.

I don't see the need for an equation question. I have tested my solution and the value in the screen-out radio and following text question are saved after a screen-out on the first page.

I agree, I don't see why it should not work, if you use group by group display with both questions on the same page.

If you use question by question, then the additional question would not be shown if the zero quota is on the first question. Which is why I mentioned the option of the equation question after these two questions. Just an alternative.