Problem pointing subdomain to domain

A customer has a domain called forum.somedomain.com but want's to change this to http://www.somedomain.com/forum and not use the subdomain link anymore.
So we deleted the subdomain, but without removing the files. This way the directory /forum stays in tact, but the subdomain dissapears.

Now I searched the forums but could not find a good solution.
Every solution is giving "apache is functioning normally".

So I thought maybe it was because of the removal of the subdomain. I created forum.somedomain.com as a domain for the user.
Then I created a redirect from within DA from http://forum.somedomain.com/ to site http://www.somedomain.com
I want it pointed to the root of this somedomain.com domain, because there will be a notice and a redirect there, which is working properly.

Still I get the "apache is functioning normally".

What am I doing wrong? How can this be fixed?