Is it OK to review a manuscript written by an editor who handled (and accepted) one of your papers? I have been invited to review a manuscript written by several authors. One of them has been the editor of one of my recently published papers. This is the only interaction I have ever had with the author of the manuscript.

Would it be OK for me to accept the review request?

While I personally do not see anything wrong with it, it might be the case that I am positively biased towards the paper, without realising it. Hence my question.

Bonus question: the journal adopts an open peer review system, where the reviews and the reviewers' names can be published alongside the paper. Shall I accept the review request, would it make sense to disclose my concern in the review I submit? Assuming that the paper gets accepted, of course.
Short answer: yes it is ok to accept the review.

Long answer: Yes, it is (still) ok to accept the review. If you can only review paper written by authors you have never ever interacted with, at some point in your career, you'll stop reviewing. Especially if you are working in a small field of research. For me, in your case, it is more than fine to accept the review. The question becomes more tricky if it is from someone you work(ed) with...

If you want to try out some really tricky reviewing scenario, try out this test: http://blogs.biomedcentral.com/bmcblog/2015/08/21/quiz-can-navigate-tricky-peer-review-scenarios/