[ejabberd] mod_muc fails and ejabberd doesn't start

[Vieri]

Hi,

How can I pinpoint the configuration or installation error that produces the following message?

What does "undef" refer to? What have I forgotten to define?

C(<0.38.0>:gen_mod:75) : Problem starting the module mod_muc for host "localhost"
 options: [{access,muc},
           {access_create,muc_create},
           {access_persistent,muc_create},
           {access_admin,muc_admin}]
 error: undef

=ERROR REPORT==== 2012-04-02 15:43:01 ===
C(<0.38.0>:gen_mod:80) : ejabberd initialization was aborted because a module start failed.

Thanks,

Vieri