Hi Possum. It's not a server error message. Reason it comes up with that page is that your Web content is arranged in sub-directories and you don't have a landing page in the root of your domain. The quickest way to get rid of that Web-directory page is to create a landing page (index.html) in the root of your domain so that when people use your domain url only, then it will open as a Web page instead of showing your directories.
If you don't have content for a landings page you could move the content of your blog directly into the root of your domain so that the domain opens as your blog. You could create/use a static page in your blog that would be the landing page and then in the menu bar create a link to your blog and to your Forum. Another option that people use is to create an .htaccess file to redirect the main domain to the sub-directories. I'd prefer however to have either a landings page or have the blog open up in the main domain. It's better for SEO as well.
Rule is never to have zipped folders - like always delete them immediately after you've unzipped them.
Here is a tutorial for how to move your blog to the root directory of your domain so that if people use the main domain that it will automatically open up as your blog:
1. First make a backup of everything - download the backup to your desktop. Also make a separate backup of your database.
2. Open your WordPress Dashboard to prepare the URL change. Go to Settings/General. Look for the URL domain.com/wordpress. There are two instances one after the other - one for WordPress and the other for Site. You need to change both to domain only by deleting the /wordpress at the end. Then save the settings and log out of the WP Dashboard.
3. You then need to go to the Web File Manager (Filezilla?) - copy/cut the content inside the WordPress folder and paste them in the root folder of the main domain. You will then have an empty WordPress folder that you can delete as the content of that folder will now be in the root public_html folder.
Tip: When you change the URL make sure you delete all cookies and cache before you start the domain again. For some ISPs it may take a bit of time for the domain changes to propagate.
Side note not exactly to do with your question but i noticed your using apache on the raspberry pi, I would very highly recommend using nginx instead it is a lot lot less bloated on memory which makes it perfect for the pi. I setup an nginx webserver on my pi so that i can control light switches in my house and it works great.
There are some good tutorials out their if you want i can find the one i followed to help you install nginx, and with nginx by default it doesnt show a list of files when there is no index.php / html it shows a 403 forbidden page.
andy26's recommendation is a good one. There are more lightweight alternatives to apache like nginx and lighttp, specially if running in a RaspPi.