If your $self->{'cfg'}->param("paths.sessions") is different from the default (/tmp/), it will try to read the sessions from a different directory as where the previous sessions are stored.

Also did you check that the previous sessions created files when you stored them?

Update: since a session ID of undef makes CGI::Session creating a new one, you can omit the condition and simply call something along these lines:

$self->{'s'} = CGI::Session->new(
               'driver:file;serializer:default;id:md5',
               $self->{'q'},
               { Directory => $self->{'cfg'}->param("paths.sessions") 
+},
);
[download]

(untested)