The other day around a bumped into the following error message displayed in the web browser console in the Episerver edit mode like the link below:
The solution suggested that the custom errors section of the web.config was misused and should be taken care of.
In my case it was not the web.config's fault - it was a conflicting page type in the database entry.
I ended up by going into the sql for the table [dbo].[tblContentLanguage] and retrieve the conflicted page type by delete commands in sql and delete the entries reflecting this.
By then I was able to recreate my page type in Episerver without having the Dojo 409 error in the front end.
Most likely, you know on which segment in the tree the conflict occures and may create your own select and delete query on the table.