relative path of my forum is /web/htdocs/forum.mysite.com/home and NOT /web/htdocs/www.mysite.com/home/forum.
That would be
absolute paths and not
relative ones. However, I strongly suggest to check with your webhost if that is
actually the case - I strongly doubt that your webspace is internally (on operating system and file system level) set up in the way you describe, as the maintainer of your webhost would have to keep track of two seemingly unrelated paths for one account.
Are you sure that the absolute path on file system level actually is
/web/htdocs/forum.mysite.com/home ? Please ask your webhost. Additionally, stop using placeholders in your postings: you can not be the owner of the domain
mysite.com, so don't use that placeholder, but the real data.
If your webhost should give you two absolute paths (one for your webroot, one for your forum's sub-domain) and you can not determine the relative path, that would be fine as well: post those absolute paths (the real ones) and we'll certainly be able to tell you the real relative ones.
If
relative path of my forum is /web/htdocs/forum.mysite.com/home and NOT /web/htdocs/www.mysite.com/home/forum.
actually is the case then you have one of the rare setups where you can't bridge due to the way cookies work. In that case, complain at your webhost.