At first it was a timezone/cookie issue for me. Then once I solved that by adding the hack to the login file (to set it to always auto-login) that went away. And for some reason just this one user's account wasn't logging in.
I checked everything over:
- server timezone
- punbb timezone
- computer timezone
etc
Nothing. Fixing the User_id to a valid one made the problem go away.
In your case @frankpeng try the method outlined here: