Not really. I don’t know all of the sequence of events that happens. If I was going to do this I would start at the beginning where the post is submitted to, this would be the file /wp-admin/admin.php. I’d then end up doing a lot of searching and figuring out what happens next. I would edit code and put in
echo '<pre>'; print_r($some_array); die;
or
echo $some_value; die;
so that I can see what’s happening along the way. I would keep progressing further through the code. and I would hope that eventually I saw something, or came across some reason as to why the redirect was causing a 404. More than likely at some point I’d start making alterations to values to see if I could stop it from happening.
Debugging is mostly guesswork once you get past the “where to start” question.
You could also reverse that and start at the end, sometimes starting at the end can get you further than starting at the beginning. I think, but I’m not 100% sure, that the redirect happens on line 236 of /advanced-custom-fields-pro/pro/admin/options-page.php
