Thank you for your help. I’ve already sent a request to the hosting provider.
Only plugin is DMS guestbook. Your other options would imply a malicious hack? Can I modify .htaccess to fix this? I’ve copied its contents below.
One last question: why does the 3.0+ versions of wordpress use this /blogs.dir/1/ structure? Old structure (uploads folder organized by year and month) seemed much more straightforward.
.htaccess existing contents:
RewriteEngine On
RewriteBase /
#uploaded files
#RewriteRule ^(.*/)?files/$ index.php [L]
#RewriteCond %{REQUEST_URI} !.*wp-content/plugins.*
#RewriteRule ^(.*/)?files/(.*) wp-includes/ms-files.php?file=$2 [L]
# add a trailing slash to /wp-admin
RewriteCond %{REQUEST_URI} ^.*/wp-admin$
RewriteRule ^(.+)$ $1/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule . – [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-.*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
<IfModule mod_security.c>
<Files async-upload.php>
SecFilterEngine Off
SecFilterScanPOST Off
</Files>
</IfModule>