[squidsk]

You didn't answer what account your web software, presumable apache, runs on.

Run the command:

ps aux | egrep '(apache|httpd)'

The account associated with that process needs to be the one that has write access to your web files or uploads won't work.

For example, on standard ubuntu the web server software runs with the www-data account. If that account does not have write access to your forum folder you won't be able to upload into that folder.