the log for this error
Started GET "/hostgroups" for 192.168.8.1 at Thu Apr 04 16:57:22 +0200 2013
Processing by HostgroupsController#index as HTML
Rendered hostgroups/welcome.html.erb within layouts/application (1.0ms)
Read fragment views/tabs_and_title_records-1 (0.4ms)
Rendered home/_topbar.html.erb (2.9ms)
Filter chain halted as :welcome rendered or redirected
Completed 200 OK in 16ms (Views: 9.7ms | ActiveRecord: 0.9ms)
Started GET "/hostgroups/new" for 192.168.8.1 at Thu Apr 04 16:57:24 +0200 2013
Processing by HostgroupsController#new as HTML
Rendered common/_domain.html.erb (187.6ms)
Rendered common/_domain_subnet.html.erb (194.6ms)
Rendered hostgroups/_form.html.erb (214.6ms)
Rendered hostgroups/new.html.erb within layouts/application (215.0ms)
Operation FAILED: undefined local variable or method `freeip_subnets_path' for #<#<Class:0x7f4483185348>:0x7f448317f3f8>
Rendered common/500.html.erb within layouts/application (361.1ms)
Rendered home/_topbar.html.erb (1.7ms)
Completed 500 Internal Server Error in 753ms (Views: 366.1ms | ActiveRecord: 3.4ms)

I'm getting a similar error when editing hostgroups with 1.2:

 NameError
undefined local variable or method `freeip_subnets_path' for #<#<Class:0x007fb7286583d0>:0x007fb728652480>
app/views/common/_domain.html.erb:4:in `block in _app_views_common__domain_html_erb__4478866523255592300_70212319545820'
app/views/common/_domain.html.erb:1:in `_app_views_common__domain_html_erb__4478866523255592300_70212319545820'
app/views/common/_domain_subnet.html.erb:7:in `block in _app_views_common__domain_subnet_html_erb___565491410454354704_70212319611860'
app/views/common/_domain_subnet.html.erb:1:in `_app_views_common__domain_subnet_html_erb___565491410454354704_70212319611860'
app/views/hostgroups/_form.html.erb:43:in `block in _app_views_hostgroups__form_html_erb___1405007256948970500_70212316712040'
app/helpers/layout_helper.rb:211:in `form_for'
app/views/hostgroups/_form.html.erb:2:in `_app_views_hostgroups__form_html_erb___1405007256948970500_70212316712040'
app/views/hostgroups/edit.html.erb:3:in `_app_views_hostgroups_edit_html_erb__1498942449115908628_70212316726700'
lib/foreman/thread_session.rb:31:in `clear_thread'

will try to remove the :data-url bit in the ERB file as per your IRC suggestion