There are 2 issues here (1 problem):

1. PM Reply -- they work, but not how they did in the previous forum.  All PM replies appear under the original PM -- similar to how topics appear in the Thread.  Just click on you original PM to see the replies
2. Inbox (20) -- this is a migration problem as all PMs from the old forum are flagged as new.  If you have more then 20 PMs, it will show 20.  To fix it, remove or read all your old PMs.

Hope this helps