Well, I'm not sure what would be the problem here, but some general comments:
There have been various changes in session handling between 5.3 and 5.4, so most likely it is in there. Check if there are any references to register_shutdown_function, and replace them with session_register_shutdown.
Second, check the return value of session_set_save_handler, it is a bad habit not checking return value of a function, just because it works. If that doesn't help, take the session piece out of the big framework, and
test it in a separate file. If that also fails, you've got a small piece of code you can post here which will encourage others to have a look at it at their box.