Why do I get a runtime error when I try to create an new subsite?

I am trying to create a new community site as a subsite to a host-headed site collection.

I've specified name, valid URL, community Site template and to use the same permissions and navigation as the parent. when I click create I get this error message:

This site collection already has a blog subsite which cas crated with no problems. The only customisation that has been performed on the site-collection is the use of a custom masterpage and stylesheet.

I've tried altering the web.config file in the way the error message suggests and it had no effect. There are no new events in the Event Log after attempting the create.

Does anyone know why this is happening and how to fix it?

Update: It appears that I only get this error when creating subsites with the Community Site template. This suggests a missing dependancy but I have activated both Sharepoint Server Standard Site Collection Features and Community Site Feature. Is there anything I've missed?