[chef] Error when starting chef-server

Hi,

I have the following error when I start chef-server on my RHEL 5 :

<internal:lib/rubygems/custom_require>:29:in `require': no such file to load 
-- yajl/yajl (LoadError)
        from <internal:lib/rubygems/custom_require>:29:in `require'
        from /usr/lib64/ruby/site/1.9.1/gems/yajl-ruby-0.8.1/lib/yajl.rb:2:in 
`<top (required)>'
        from <internal:lib/rubygems/custom_require>:33:in `require'
        from <internal:lib/rubygems/custom_require>:33:in `rescue in require'
        from <internal:lib/rubygems/custom_require>:29:in `require'
        from 
/usr/lib64/ruby/site/1.9.1/gems/ohai-0.6.4/lib/ohai/system.rb:25:in `<top 
(required)>'
        from <internal:lib/rubygems/custom_require>:29:in `require'
        from <internal:lib/rubygems/custom_require>:29:in `require'
        from /usr/lib64/ruby/site/1.9.1/gems/ohai-0.6.4/lib/ohai.rb:21:in 
`<top (required)>'
        from <internal:lib/rubygems/custom_require>:29:in `require'
        from <internal:lib/rubygems/custom_require>:29:in `require'

I checked the forum archives and found only one occurence of this issue which 
seem to be related to gecode but gecode is installed : 

Loaded plugins: rhnplugin, security
This system is not registered with RHN.
RHN support will be disabled.
Installed Packages
gecode.x86_64                                                                 
                              3.5.0-1.el5                                     
                                                           installed

Any idea?

Thanks,
Vina.