If this continues, my instance may need to defederate from Lemmy.ml.
That probably isn't the right fix for the long term, because otherwise it leaves kbin vulnerable to being DoSed by another instance intentionally returning 403s, or breaking down if something goes wrong with a lemmy or kbin instance that causes it to throw errors for totally innocent reasons. Kbin should be fixed to handle getting 403s reasonably-gracefully.
EDIT: Also, might be a good idea to check lemmy's handling of getting 403s as well for the same reason.
EDIT2: I don't know if lemmy/kbin have any kind of test code to test how well they deal with federated servers malfunctioning, but my guess is that using one set of tests for both would probably save effort.