A wild stab in the dark.

When I had a similar problem with CGI::Application::Plugin::Session I was able to trace it back to not having the right version of Scalar::Util. Recent versions of which have a refaddr method which is used by recent versions of CGI::Session. I think it was at the time serialising objects became possible which makes some sense.

S::U is not a pure Perl module so I wasn't able to 'install' (copy) the module to my hoster. A bit of digging showed that installing an older version of C::S that doesn't use Scalar::Util->refaddr was a possible way forward. I haven't tried it myself because by this time I had found another work around.

The horrible story: CGI::Session/MySQL dependancy/version woes.
Updated: Added link.