Yes, I'm redirecting the target of a post. And it does seem to be a size issue; the form is dynamically generated and smaller versions of the form are redirected just fine by the action page, but larger ones (not THAT large, either -- 1.3 K, about) exhibit the problem I described. ad_returnredirect doesn't help, unfortunately. Although util_ReturnMetaRefresh DOES work, but this is so ugly I'd rather not do it.
