Most likely you have a post or page using the same slug, or you have forums in the trash that have the same name, empty the trash if thats the case.
Dear sir
there is no any forum in the Trash !
if you allow me then I will send you the admin detail.
You will have a slug conflict from some other post, or page, or topic.
Dear sir
I am unable trace other pages/topics/replies with same name !
I tried by searching from database and by normel search query, but nothing found.
One thing I noticed that I have seen an attachment with same slug of forum name.
Yes sir, problem has resolved now.
infact there was some attachments with same slug and I changed them.
