@PeekaBooz can't you post the full error message?
I expect you're using ubuntu or debian, so in the meantime what you can do, to be sure everything you need is 775, just do recursively adjust that like this:
chown -R www-data:www-data /path/to/your/folder/forum #Change user:group to www-data:www-data
chmod -R 775 /path/to/your/folder/forum #change permission to 775 recursively
