Maybe you made something wrong while configuring your Cname.Â
I advise you to start from the beginning. There are three information to enter 1- Time to live:
Indicate your users will get in cache your pages. Choose a long period of time (around 15 days is enough)
2- Name: It is the particle is the domain. For example: name.your_domain.uk
If you want to have your forum on www.your_domain.uk, in that case the name =www If you want to have your forum on forum.your_domain.uk, in that case the name = forum.... etc
3- Value: It is the url targeted, the url of your forum