My problem solved permanently by simply doing this:
Adding these three lines ABOVE the wordpress htaccess settings:
RewriteCond %{HTTP_HOST} wiki.mydomain.com
RewriteCond %{REQUEST_URI} !wiki/
RewriteRule ^(.*)$ wiki/$1 [L]
(of course below these:
RewriteEngine On
RewriteBase /
)
By saying wordpress htaccess settings, I mean:
# BEGIN WordPress
RewriteRule ^index\.php$ - [L]
# uploaded files
RewriteRule ^files/(.+) wp-includes/ms-files.php?file=$1 [L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule . index.php [L]
# END WordPress