The basic issue is that the good Mikrotik guys like to call “stable” something that is usually very unlike it, It is more like a “feature freeze” release, but from that to have a “stable” in the sense of “reliable” OS, there is a long way.
Ideally you should have filed a support ticket at the official Mikrotik support, but since you already solved the issue by downgrading to an earlier version, you are probably not in the condition to reproduce the issue and provide all the needed details, supout, logs, etc.
You could still post a more detailed report (add as many details as you can) of what happened on the main “release thread” here:
http://forum.mikrotik.com/t/v7-15-3-stable-is-released/176334/1
where it is more likely that some Mikrotik developer may notice it.